ALSA: seq: Reorganize kconfig and build

This is a slightly intensive rewrite of Kconfig and Makefile about
ALSA sequencer stuff.

The first major change is that the kconfig items for the sequencer are
moved to sound/core/seq/Kconfig.  OK, that's easy.

The substantial change is that, instead of hackish top-level module
selection in Makefile, we define a Kconfig item for each sequencer
module.  The driver that requires such sequencer components select
exclusively the kconfig items.  This is more straightforward and
standard way.

Signed-off-by: Takashi Iwai <tiwai@suse.de>