ARM: support XZ compressed kernels

Wire up support for the XZ decompressor

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>

Conflicts:
	arch/arm/boot/compressed/Makefile