cmd: mvebu/bubt: Fix parsing SATA kwbimage

Despite the official specification, Marvell BootROM does not interpret
srcaddr from SATA image as number of sectors the beginning of the hard
drive, but as number of sectors relative to the main header.

Reject invalid and accept valid SATA images.

Fixes: 5a065349 ("cmd: mvebu/bubt: Check for A38x image data checksum")
Signed-off-by: Pali Rohár <pali@kernel.org>