ffmpeg: consolidate duplicated config options

Commit 62ab07ef added a bunch
of hard-coded options.  Some of these options are then again
added (or negated) based on kconfig settings.  This patch removes
the hard coded swscale and postproc settings, as they are enabled
or disabled later.  It also moves the --disable-pic lower
as an else clause to the part that adds --enable-pic.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>