base-files: fix a preinit hang in jffs2_ready() when no rootfs_data partition is found (#7362)

SVN-Revision: 21511

package/base-files/files/lib/functions/boot.sh

@@ -32,6 +32,7 @@ find_mtd_part() {
 
 jffs2_ready () {
     mtdpart="$(find_mtd_part rootfs_data)"
+    [ -z "$mtdpart" ] && return 1
     magic=$(hexdump $mtdpart -n 4 -e '4/1 "%02x"')
     [ "$magic" != "deadc0de" ]
 }