tools: kwbimage: Add support for XIP SPI/NOR images

tools: kwbimage: Add support for XIP SPI/NOR images

Marvell BootROM can execute SPI images directly from NOR (either SPI/serial
or parallel) without copying them to DDR RAM. This is know at XIP - execute
in place. To achieve that, destination address in kwbimage must be set to
0xFFFFFFFF and execute address to the offset in bytes from the beginning of
NOR memory.

Kirkwood and Dove which use kwbimage v0 format and have SPI address space
mapped to physical memory at 0xE8000000-0xEFFFFFFF by BootROM.

Armada SoCs use kwbimage v1 format and have SPI address space mapped to
physical memory at 0xD4000000-0xD7FFFFFF and Device bus address space (used
for parallel NOR) at 0xD8000000-0xDFFFFFFF.

Add support for generating XIP kwbimages by mkimage -x flag and mark xflag
as valid option in kwbimage.c.

Signed-off-by: Pali Rohár <pali@kernel.org>