[][OpenWrt Dev][Enable configurable rootfs dev for UBI]

[Description]
Add support for UBI to use volumes other than "rootfs" for default rootfs
device for mounting root.
Configuration can be done by setting kernel commandline.

[Release-log]
N/A

Change-Id: Ie23517423970955e1d1b91a297c7cc3b70263663
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6105792
1 file changed