V4L/DVB (4518): Allow manually selecting the proper helper drivers

There are several boards on V4L that can work with several different
helper modules for audio/video encoding/decoding and similar stuff.
This patch adds the capability of choicing between autoselecting the
pertinent helper modules for each driver, or to manually selecting
them.
Acked-by: Michael Krufky <mkrufky@linuxtv.org>
Acked-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>