[][Add initial mt7981 target support in SDK]

[Description]
Add initial mt7981 target support in SDK

[Release-log]
N/A

Change-Id: Id7b3e04856c2a26e030a83dbfb57779b815b173f
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5058341
11 files changed