gadged: thor: add board signature check when download 'u-boot-mmc.bin'

gadged: thor: add board signature check when download 'u-boot-mmc.bin'

This patch adds checking the special signature of downloaded U-Boot binary.
The new function check_board_signature() is called before do dfu_write()
in thor gadged code.

The board signature is checked for:
- the running U-Boot
- downloaded 'u-boot-mmc.bin'
at offset of binary start defined by:
- (CONFIG_SIGN_IMAGE_SIZE - HDR_SIZE)

The download can succeed when signatures are equal.

The default U-Boot image size defined by CONFIG_SIGN_IMAGE_SIZE is 1MB.

Other changes:
- print info about running/downloading U-Boot signature
- print info about wrong image signature/size
- don't allow downloading unsigned or signed wrong U-Boot image
- allow download any image if no signature found at running U-Boot

The first version added by:
Signed-off-by: Inha Song <ideal.song@samsung.com>
Upgrade:
Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>