Python: add external modules option

all external python packages/libraries should reside in
"external python modules" section.

Move existing modules python-serial and python-mad there

Move python-mad from package/multimedia to package

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>