docs-rst: media/Makefile: reorganize the rules

Better organize the media/Makefile, in order to better
split what's related to image conversion from the ones
related to parse-headers.pl.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>