sound: make OSS sound core optional

sound/sound_core.c implements soundcore.ko and contains two parts -
sound_class which is shared by both ALSA and OSS and device
redirection support for OSS.  It's always compiled when any sound
support is enabled although it's necessary only when OSS (the actual
one or emulation) is enabled.  This is slightly wasteful and as device
redirection always registers character device region for major 14, it
prevents alternative implementation.

This patch introduces a new config SOUND_OSS_CORE which is selected
iff OSS support is actually necessary and build the OSS core part
conditionally.

If OSS is disabled, soundcore merely contains sound_class but leaving
it that way seems to be the simplest approach as otherwise sound_class
should be in ALSA core file if OSS is disabled but should be in
soundcore if OSS is enabled.  Also, there's also the user confusion
factor.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>