[][OpenWrt Dev][Add support for dual-boot on UBI devices]

[Description]
Add support for dual-boot on UBI devices
Currently supported on mt7986

[Release-log]
N/A

Change-Id: I32e0f744baaacd9bb094c0764ccfdb2048ad798b
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6105794
2 files changed