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>
diff --git a/doc/README.kwbimage b/doc/README.kwbimage
index 8ed708c..13f6f92 100644
--- a/doc/README.kwbimage
+++ b/doc/README.kwbimage
@@ -42,7 +42,7 @@
 	kwbimage.cfg.  The name can be set as part of the full path
 	to the file using CONFIG_SYS_KWD_CONFIG (probably in
 	include/configs/<yourboard>.h).   The path should look like:
-	$(SRCTREE)/$(CONFIG_BOARDDIR)/<yourkwbimagename>.cfg
+	$(CONFIG_BOARDDIR)/<yourkwbimagename>.cfg
 2. This file can have empty lines and lines starting with "#" as first
 	character to put comments
 3. This file can have configuration command lines as mentioned below,