tools: kwboot: 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.

To parse SATA kwbimage in the same way as Marvell BootROM, it is needed to
interpret srcaddr as relative offset to the main header. This change fixes
loading of SATA images via kwboot over UART.

Fixes: 792e4235 ("tools: kwboot: Patch source address in image header")
Signed-off-by: Pali Rohár <pali@kernel.org>