gst-plugins-base: make alsa plug-in configurable

It may be desirable not to use the alsa sink for playback
even though alsa-lib is installed. So make this a configuration
option. This also takes care of selecting the proper options
for alsa-lib
(BR2_PACKAGE_ALSA_LIB_MIXER and BR2_PACKAGE_ALSA_LIB_PCM).

[Peter: fix indentation]
Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>