OpenWrt's support for splitting rootfs (to create an extra "rootfs_data"
partition) is limited to partitions called "rootfs". Upstream kernel
allows any name partition to be rootfs if it has "linux,rootfs" property
set. Add split support to such partitions in OpenWrt code.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit e4770db163)