kirkwood: kwbimage: refactor CONFIG_SYS_KWD_CONFIG

kirkwood: kwbimage: refactor CONFIG_SYS_KWD_CONFIG

Pull out "$(SRCTREE)/" from CONFIG_SYS_KWD_CONFIG
and push it into the top Makefile.

Signed-off-by: Masahiro Yamada <[email protected]>
Cc: Michael Walle <[email protected]>
Cc: Simon Guinot <[email protected]>
Cc: Dave Purdy <[email protected]>
Cc: Stefan Herbrechtsmeier <[email protected]>
Cc: Luka Perkov <[email protected]>
Cc: Valentin Longchamp <[email protected]>
Cc: Jason Cooper <[email protected]>
Cc: Siddarth Gore <[email protected]>
Cc: Prafulla Wadaskar <[email protected]>
Cc: Eric Cooper <[email protected]>
Cc: Suriyan Ramasami <[email protected]>