btrfs-progs: add "btrfs device scan" before overlay/extroot

btrfs-progs: add "btrfs device scan" before overlay/extroot

If users choose to build OpenWrt with btrfs included
and want to use a btrfs RAID for overlay/extroot,
then devices need to be scanned *before* mounting overlay/extroot.
If not, btrfs won't find all RAID drives and fail to mount.

This commit:
- creates a duplicate (symlink) of the btrfs scan script so that it runs
  both before and after overlay/extroot mount,
- changes the scan command to not depend on blkid (-d),
- outputs the scan results to kernel log to show what's being detected,
  as the system logger is not yet running.

Signed-off-by: Marius Dinu <[email protected]>