kernel: move old "rootfs" splitter into separated file

This will simplify removing it in the future.

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


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42828 3c298f89-4303-0410-b956-a3cf2f4a3e73