kirkwood: include ext4 support by default

Several users reported that they would like this feature.

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38901 3c298f89-4303-0410-b956-a3cf2f4a3e73