scripts: Update mkknlimg, just in case

With the removal of the vc_cma driver, mkknlimg lost an indication that
the user had built a downstream kernel. Update the script, adding a few
more key strings, in case it is still being used.

Note that mkknlimg is now deprecated, except to tag kernels as upstream
(283x), and thus requiring upstream DTBs.

See: https://github.com/raspberrypi/linux/issues/2239



Signed-off-by: Phil Elwell <phil@raspberrypi.org>