Move MALTA_COMMON to std.MALTA (as we have for XLP and SWARM).

This avoids universe trying to build MALTA_COMMON and fail due to
the problem addressed in r276773.
Include std.MALTA from the MALTA mipsel and MALTA64 mips64el config files,
where the machine lines exist.

sys/mips/conf/MALTA

@@ -4,7 +4,7 @@
  
 ident		MALTA
 
-include		"MALTA_COMMON"
+include		"std.MALTA"
  
 machine		mips mipsel	# Malta supports both, so it isn't in std.malta
  

sys/mips/conf/MALTA64

@@ -4,7 +4,7 @@
  
 ident		MALTA64
 
-include		"MALTA_COMMON"
+include		"std.MALTA"
  
 machine		mips mips64el	# Malta supports both, so it isn't in std.malta