kernel: revert r24160 (remove the bogus ext2/ext3 dependency on linux 2.6.30/31)

kernel: revert r24160 (remove the bogus ext2/ext3 dependency on linux 2.6.30/31)

For kernel versions newer then 2.6.31 the ext4 module can be used to mount
ext2/3 filesystems.
Building ext2/3 as modules on the other hand breaks using ext4 for mounting ext2
or ext3, which breaks booting from ext2/3 on machines where the ext4 module is
built into the kernel.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@26645 3c298f89-4303-0410-b956-a3cf2f4a3e73