cmd: gpt: backup boot code before writing MBR

cmd: gpt: backup boot code before writing MBR

On some cases the first 440 bytes of MBR are used to keep an additional
information for ROM boot loader. 'gpt write' command doesn't preserve
that area and makes boot code gone.

Preserve boot code area when run 'gpt write' command.

Signed-off-by: Vincent Tinelli <vincent.tinelli@intel.com>
Signed-off-by: Brennan Ashton <brn@deako.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>