btrfs-progs: use libattr

OpenWrt has by default no sys/xattr.h
use libattr instead

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>
btrfs-progs: disable btrfs-convert

- using btrfs-convert makes less sense because
ressources are limited
- avoid handling other includes by disabling it

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>