package/squashfs: add selection for the host variant

If any of the post-image scripts wants to handle squashfs filesystems,
we need to expose an option for squashfs-tools to be user-selectable.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ryan Barnett <rjbarnet@rockwellcollins.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>