UBOOT中增加backtrace功能

目前有些bootloader中编译时，没有设置fp寄存器，因此无法打印出backtrace，调试时会较为麻烦，按照如下方法，可以在异常时增加backtrace打印，更加方便debug。

1. arch/arm/cpu/armv7/config.mk中加入编译参数-mapcs-frame

PLATFORM_CPPFLAGS += $(call cc-option, -mapcs-frame,)

2. 异常函数中加入如下内容，主要作用是打印backtrace

struct stackframe {
/*
   FP member should hold R7 when CONFIG_THUMB2_KERNEL is 
   enabled and R11 otherwise.
*/
    unsigned long fp;
    unsigned  long sp;
    unsigned long lr;
    unsigned long pc;
}

#define MAX BACK _TRACE LEVEL 6
/*
   Unwind a single frame starting with *sp for the symbol at *pc. 
   It updates the *pc and tsp with the new values。
*/

int unwind_ frame(struct stackframe *frame)
{
    static int cnt=0;

    unsigned long fp = frame->fp;
    if(++cnt = MAX_ BACK_TRACE_LEVEL)
        return -1:

    frame->fp = *(unsigned long *)(fp - 12);
    frame->sp = *(unsigned long *)(fp - 8);
    frame->lr = *(unsigned long *)(fp - 4);

    return 0;
}

void dump_backtrace_entry(unsigned long where, unsigned long from, unsigned long frame)
{
    printf("Function entered at [<%08lx>] from [<%08lx>]\n",where, from);
}

void dump stack(struct pt_regs *regs)
{
    struct stackframe frame ;
    int urc;

    frame.fp = regs->ARM_fp;
    frame.sp = regs->ARM_sp;
    frame.lr= regs->ARM_lr;
    frame.pc = instruction_pointer (regs):

    printf("Function entered at [<%08lx>] from [<%88lx>]n", frame.pc, frame .lr);

    while(1)
    {
        unsigned long where = frame.lr :

        urc = unwind. frame(&frame);
        if (urc< 0)
            break;

        dump_ backtrace entry(where, frame.lr, frame.sp - 4);
    }
}