dist/mkfile: simplify

sys/lib/dist/mkfile

@@ -1,11 +1,14 @@
-cd:V: /tmp/9front.iso
+proto=/n/src9/sys/lib/sysconfig/proto/cdproto
+ini=pc/plan9.ini.cd
+iso=/tmp/9front.iso
+
+cd:V:	$iso
 	
 
-%.iso: pc/plan9.ini.cd
-	@{	rfork n
-		bind usr /n/src9/usr
-		bind /n/src9 /n/src9/dist/plan9front
-		bind pc/plan9.ini.cd /n/src9/cfg/plan9.ini
-		disk/mk9660 -c9j -B 386/9bootiso -p /n/src9/sys/lib/sysconfig/proto/cdproto \
-			-s /n/src9 -v 'Plan 9 Front' $target
+%.iso:  $proto $ini
+	@{rfork n
+	bind usr /n/src9/usr
+	bind /n/src9 /n/src9/dist/plan9front
+	bind $ini /n/src9/cfg/plan9.ini
+	disk/mk9660 -c9j -B 386/9bootiso -p $proto -s /n/src9 -v 'Plan 9 Front' $target
 	}