image_types.bbclass: Replace squashfs-lzma with squashfs-xz

image_types.bbclass: Replace squashfs-lzma with squashfs-xz

Booting an image generated with squashfs-lzma results in a kernel
error: "Filesystem uses "lzma" compression. This is not supported".

Currently (well at least in Linux 3.8) the officially supported
decompressors are:
* LZO
* XZ
* ZLIB

This change makes sure we use a supported compression algorithm for
squashed root filesystems.

Signed-off-by: Marc Ferland <ferlandm@sonatest.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>