Make "default" PCM card-specific

"default" PCM can be card-specific.
The definition is moved to pcm/default.conf, and read on-demmand via
aliases.conf.  The card config can override the default config.

src/conf/pcm/Makefile.am

@@ -1,4 +1,4 @@
-cfg_files = front.conf rear.conf center_lfe.conf side.conf\
+cfg_files = default.conf front.conf rear.conf center_lfe.conf side.conf\
 	    surround40.conf surround41.conf \
 	    surround50.conf surround51.conf \
 	    surround71.conf iec958.conf modem.conf

src/conf/pcm/default.conf

@@ -0,0 +1,39 @@
+#
+# Default output
+#
+
+pcm.!default {
+	@args [ CARD ]
+	@args.CARD {
+		type string
+		default {
+			@func getenv
+			vars [
+				ALSA_PCM_CARD
+				ALSA_CARD
+			]
+			default {
+				@func refer
+				name defaults.pcm.card
+			}
+		}
+	}
+	# use card-specific definition if exists
+	@func refer
+	name {
+		@func concat
+		strings [
+			"cards."
+			{
+				@func card_driver
+				card $CARD
+			}
+			".pcm.default:CARD=" $CARD
+		]
+	}
+	default {
+		# use plughw as default
+		type plug
+		slave.pcm hw
+	}
+}