[][OpenWrt Dev][Add boot param for reserving rootfs_data]

[Description]
Add boot param for reserving rootfs_data during sysupgrade

[Release-log]
N/A

Change-Id: I0b8d16162c714db39dfd545e0fe225cfd5ced832
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6105788
1 file changed