Merge branch 'alsa_pcm_for_firefox' into 'master'

component/alsa: Do not disable pcm, firefox needs it otherwise it crashes when playing sounds

In a selenium suite for a project, we have some gadgets that play some sounds using html5 audio element.
We discovered that the firefox coming with SlapOS used in test nodes crashes as soon as it plays a sound, with this error:
```
parts/firefox/firefox: relocation error:
/srv/slapgrid/slappart11/srv/runner/software/88ef27d24c6e0a792f72f568146a3838/parts/firefox/libxul.so:
symbol snd_pcm_open, version ALSA_0.9 not defined in file libasound.so.2 with link time reference
```
indeed, libasound.so does not contain this symbol if built with --disable-pcm

Building libasound without this switch fixed this problem and did not seem to cause any trouble, but I have no idea why pcm was disabled in the first place. It has been like this since initial commit of this component profile at 82900a33


It's not urgent, but we would like to have this in testnode branch at some point. Shall I make another merge request for this commit in testnode branch or just merge master in testnode branch ?

See merge request !35

component/alsa/buildout.cfg

@@ -11,7 +11,6 @@ configure-options =
   --disable-static
   --disable-aload
   --disable-mixer
-  --disable-pcm
   --disable-rawmidi
   --disable-hwdep
   --disable-seq