wic: squashfs partition support

wic: squashfs partition support

It is possible to instruct wic to create a squashfs partition by setting
--fstype=squashfs in *.wks. For now this is only useable for rootfs
partitions (note that you must have squashfs support in the kernel). An
attempt to create an empty partition will produce a warning.

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>