ALSA: clean up the logic for building sequencer modules

Instead of mangling the CONFIG_* variables in the makefiles over and
over, set a few helper variables in Kconfig.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/core/Kconfig b/sound/core/Kconfig
index 7bbdda0..6061fb5 100644
--- a/sound/core/Kconfig
+++ b/sound/core/Kconfig
@@ -205,3 +205,5 @@
 
 config SND_VMASTER
 	bool
+
+source "sound/core/seq/Kconfig"