read NAND / enable serial debugging

[xiangfu]

 i change the code like this:
--------------------------------------
#define ADDR  ((volatile unsigned *)&buf)

int start_kboot()
{
  asm volatile("mov pc, %0\n"
       :                      /* output */
       :"r"(ADDR)       /* input */
       );
  return 0;
--------------------------------------
and this .s file changed :

       ldr     r3, .L3
#APP
        mov pc, r3
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i think the assembly code is right.
but the led still not blink.