vendor.sh: further simplify config handling

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/option.sh

@@ -121,6 +121,7 @@ EOF
 
 check_defconfig()
 {
+	[ -d "$1" ] || $err "Target '$1' not defined."
 	for x in "${1}"/config/*; do
 		[ -f "${x}" ] && printf "%s\n" "$x" && return 1
 	done

include/vendor.sh

@@ -394,10 +394,7 @@ patch_rom()
 {
 	rom="${1}"
 
-	check_defconfig "$boarddir" && $err "patch_rom $boarddir: no configs"
-
-	set -- "${boarddir}/config/"*
-	. "${1}" 2>/dev/null
+	. "$(check_defconfig "${boarddir}")" 2>/dev/null || exit 0
 
 	[ "$CONFIG_HAVE_MRC" = "y" ] && \
 		inject "mrc.bin" "${CONFIG_MRC_FILE}" "mrc" "0xfffa0000"