- sclk initialization. this avoid long long sleeps and accelerate booting
+ increase buffer for printf. thia avoid the buffer overflow that corrupt the other data
* read nand_sector read_byte function call replace with directly data fetch.
  this accelarate linux kernel loading
- nand_check_badblock called before every block read. if this generate error than
  linux kernel can't be loaded. this also increase bootspeed
* define variables as static. otherwise in debug mode the kernel command
  line is corruped and linux kernel can't mount rootfs