Makefile: Rename u-boot-spl.kwb to u-boot-with-spl.kwb

File name with pattern u-boot-spl* is used on all places except in kwb
image for binary with SPL-only code. Combined binary with both SPL and
proper U-Boot in other places has file name pattern u-boot-with-spl*.

Make it consistent also for kwb image and rename u-boot-spl.kwb to
u-boot-with-spl.kwb as this image contains both SPL and proper U-Boot code.

Also update documentation about file name changes.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>