classes: image_types: apply EXTRA_IMAGECMD:squashfs* in oe_mksquashfs()

Since commit c991f9d6 ("image_types: Set SOURCE_DATE_EPOCH for squashfs"),
I assume, the EXTRA_IMAGECMD:squashfs* variable(s) has been ignored.
This is due to the override magic, which isn't applied to functions
called by IMAGE_CMD:<type>, but only to the IMAGE_CMD:<type> itself.

Other image types (e.g. ext*) works around this by passing the
EXTRA_IMAGECMD variable as an argument to the called function.

To do the same for oe_mksquashfs(), the number of mandatory arguments is
fixed to one (with a little logic to handle the zstd filename). This
allows passing ${EXTRA_IMAGECMD} as an argument to oe_mksquashfs(),
which makes the variable functional again.

Signed-off-by: Martin Hundebøll <martin@geanix.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>