squashfs: re-add legacy lzma support

Readd legacy LZMA support since older patched kernels might be using it.
Introduce an XZ target filesystem option for the new format.

[Peter: add a seperate xz target for squashfs-target as well]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>