tuna: add xz compression support

fix compile on tuna kernel and enable in defconfig

From: "Imre Kaloz" <kaloz@openwrt.org>
Subject: Wire up support for the XZ decompressor

Wire up support for the XZ decompressor

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I86d7cfa26edc433291ba5e9372b20e65e95fc970