NVMEM in UBI support has been present for a while. Use it to get the
MACs. Seems caldata needs to continue to be obtained through userspace.
This fixes label-mac-device as it needs a mac-address nvmem cell.
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17068
Signed-off-by: Robert Marko <robimarko@gmail.com>