[][OpenWrt Dev][Add dual-boot support for mt7981]

[Description]
Add dual-boot support for mt7981

[Release-log]
N/A

Change-Id: Ibc1342b0fd1f285c84bc38159c96cabcbcc135a7
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6105795
1 file changed