[][OpenWrt Dev][Add missing uboot-envtool config file for mt7981 sysupgrade]

[Description]
Add missing uboot-envtool config file for mt7981 sysupgrade

[Release-log]
N/A

Change-Id: If589b7bbeab0f352253a7cc986a5b4e4d2f6b757
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6187509
1 file changed