target/squashfs: cleanup, remove support for 3.x

Remove the code to build squashfs-tools for the host, since they can
now be built using package/squashfs.

We also remove support for Squashfs 3.x, because the Squashfs driver
merged inside the mainline kernel is a 4.x driver. So 3.x is only
useful for old kernels using a Squashfs patch.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>