meta-digi-arm: fix building kernel using out-of-tree

The way the kernel is built in yocto 1.8 has changed. Now the
build is done under ${WORKDIR}/build while the source code is at
STAGING_KERNEL_DIR, so we need to modify the '.config' in the build
directory instead of in the source directory.

This fixes:

kernel-source is not clean, please run 'make mrproper'
make[2]: *** [prepare3] Error 1
make[1]: *** [sub-make] Error 2

https://jira.digi.com/browse/DEL-1558

Signed-off-by: Javier Viguera <javier.viguera@digi.com>

meta-digi-arm/recipes-kernel/linux/linux-dey.inc

@@ -15,7 +15,7 @@ SRC_URI = " \
 do_configure() {
 	for i in $(echo ${WORKDIR}/*.cfg); do
 		[ "${i}" = "${WORKDIR}/*.cfg" ] && continue
-		cat ${i} >> '${S}/.config'
+		cat ${i} >> ${B}/.config
 	done
 	kernel_do_configure
 }