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 <yamada.m@jp.panasonic.com>
Cc: Michael Walle <michael@walle.cc>
Cc: Simon Guinot <simon.guinot@sequanux.org>
Cc: Dave Purdy <david.c.purdy@gmail.com>
Cc: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
Cc: Luka Perkov <luka@openwrt.org>
Cc: Valentin Longchamp <valentin.longchamp@keymile.com>
Cc: Jason Cooper <u-boot@lakedaemon.net>
Cc: Siddarth Gore <gores@marvell.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Eric Cooper <ecc@cmu.edu>
Cc: Suriyan Ramasami <suriyan.r@gmail.com>