mkimage: fit_image: Add option to make fit header align

mkimage: fit_image: Add option to make fit header align

The image is usually stored in block device like emmc, SD card, make the
offset of image data aligned to block(512 byte) can avoid data copy
during boot process.
eg. SPL boot from FIT image with external data:
- SPL read the first block of FIT image, and then parse the header;
- SPL read image data separately;
- The first image offset is the base_offset which is the header size;
- The second image offset is just after the first image;
- If the offset of imge does not aligned, SPL will do memcpy;
The header size is a ramdon number, which is very possible not aligned, so
add '-B size'to specify the align size in hex for better performance.

example usage:
  ./tools/mkimage -E -f u-boot.its -B 0x200 u-boot.itb

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
Reviewed-by: Tom Rini <trini@konsulko.com>