kernel: enable CONFIG_MTD_SPLIT_SQUASHFS_ROOT

This enables dedicated "rootfs" splitter by default. We still keep
CONFIG_MTD_SPLIT_SQUASHFS_ROOT, so the old (built-in) splitter will be
used as a fallback in case of problems.
Once we decide the dedicated partitioner works stable, we should remove
the old one completely.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42805 3c298f89-4303-0410-b956-a3cf2f4a3e73