tools: kwbimage: Add support for creating an image with no data

tools: kwbimage: Add support for creating an image with no data

This change add support for mkimage's -s option to kwbimage format. It will
create an kwbimage with empty data part of image (data part would contain
only required 32-bit checksum). mkimage's -s option is indicated by skipcpy
flag and it is basically in conflict with mkimage's -d (datafile) option.

"Empty" kwbimage with no data can still contain headers. For example it can
contain binary executable header which is copied by BootROM into L2SRAM.
This is useful for example for small images which can do not require DDR
RAM and can be run in L2SRAM (which do not require any initialization).

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