Sometimes the mmc deivce may come up later than kernel attempts to
mount rootfs, resulting kernel panic. Enable rootwait to fix it.
Reported-by: Yangyu Chen <cyy@cyyself.name>
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/15077
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>