This adds `filetype_squashfs` to the list of known filetypes and adds a
detection for squashfs files to file_detect_type(). This currently
matches on the `hsqs` start sequence of an image file.

Additionally, the newly introduced filetype is registered as the type of
the squashfs_driver which allows, for example, to mount squashfs without
the need to specify a type parameter.

This changes enable booting a squashfs with the simple `boot` command
pointing to the location (device) that holds the squashfs.

Note that booting with blspec is limited as the current squashfs driver
is not capable of handling symbolic links.

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>