Let's Split: establish rev2fliphalf subproject

Subproject to accomodate second half pad built in opposite orientation
(places TRRS jacks on opposing edges for shorter cable runs)

keyboards/lets_split/lets_split.h

@@ -7,6 +7,9 @@
 #ifdef SUBPROJECT_rev2
     #include "rev2.h"
 #endif
+#ifdef SUBPROJECT_rev2fliphalf
+	#include "rev2fliphalf.h"
+#endif
 
 #include "quantum.h"