操作日志：
起因：Nexus 7 flo 华硕平板，刷了一个CWM Recovery之后变成了砖机；
尝试1：使用CWM Recovery清除数据，开机失败
尝试2：刷入CM 11 Snapshot的zip卡刷包，并使用Recovery清除数据，开机失败
尝试3：刷入CM 11 eng自己编译的版本，并使用Recovery清除数据，开机失败
尝试4：fastboot flash boot /path/to/boot.img，并刷入 CM 11 eng自己编译的版本，并Recovery清除数据，开机失败
尝试5：使用Logcat抓日志，输出如下
D/AndroidRuntime( ): >>>>>> AndroidRuntime START com.android.internal.os.ZygoteInit <<<<<<
D/AndroidRuntime( ): CheckJNI is ON
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/core.jar (/data/dalvik-cache/system@framework@core.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/core.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/conscrypt.jar (/data/dalvik-cache/system@framework@conscrypt.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/conscrypt.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/okhttp.jar (/data/dalvik-cache/system@framework@okhttp.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/okhttp.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/core-junit.jar (/data/dalvik-cache/system@framework@core-junit.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/core-junit.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/bouncycastle.jar (/data/dalvik-cache/system@framework@bouncycastle.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/bouncycastle.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/ext.jar (/data/dalvik-cache/system@framework@ext.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/ext.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/framework.jar (/data/dalvik-cache/system@framework@framework.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/framework.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/framework2.jar (/data/dalvik-cache/system@framework@framework2.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/framework2.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/telephony-common.jar (/data/dalvik-cache/system@framework@telephony-common.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/telephony-common.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/voip-common.jar (/data/dalvik-cache/system@framework@voip-common.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/voip-common.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/mms-common.jar (/data/dalvik-cache/system@framework@mms-common.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/mms-common.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/android.policy.jar (/data/dalvik-cache/system@framework@android.policy.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/android.policy.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/services.jar (/data/dalvik-cache/system@framework@services.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/services.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/apache-xml.jar (/data/dalvik-cache/system@framework@apache-xml.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/apache-xml.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/webviewchromium.jar (/data/dalvik-cache/system@framework@webviewchromium.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/webviewchromium.jar'
E/dalvikvm( ): Could not stat dex cache directory '/data/dalvik-cache': No such file or directory
I/dalvikvm( ): Unable to open or create cache for /system/framework/telephony-msim.jar (/data/dalvik-cache/system@framework@telephony-msim.jar@classes.dex)
D/dalvikvm( ): Unable to process classpath element '/system/framework/telephony-msim.jar'
E/dalvikvm( ): No valid entries found in bootclasspath
'/system/framework/core.jar:/system/framework/conscrypt.jar:/system/framework/okhttp.jar:/system/framework/core-junit.jar:/system/framework/bouncycastle.jar:/system/framework/ext.jar:/system/framework/framework.jar:/system/framework/framework2.jar:/system/framework/telephony-common.jar:/system/framework/voip-common.jar:/system/framework/mms-common.jar:/system/framework/android.policy.jar:/system/framework/services.jar:/system/framework/apache-xml.jar:/system/framework/webviewchromium.jar:/system/framework/telephony-msim.jar'
E/dalvikvm( ): VM aborting
F/libc ( ): Fatal signal (SIGABRT) at 0x00000214 (code=-), thread (zygote)
I/ServiceManager( ): service 'media.audio_flinger' died
I/ServiceManager( ): service 'media.player' died
I/ServiceManager( ): service 'media.camera' died
这理应是数据没有清除完成导致的，于是 Google下，尝试使用 fastboot -w 来清除数据，最后成功开机！
小结，Nexus 7 此类问题解决方法(黄背景的是重点，前两步可选)：
adb sideload /media/Source/CyanogenMod/out/cm-11.0/target/product/flo/cm-11-20141224-UNOFFICIAL-flo.zip
fastboot flash boot /media/Source/CyanogenMod/out/cm-11.0/target/product/flo/boot.img
fastboot -w