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/keymaps/serial/config.h

@@ -26,4 +26,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #endif
 #ifdef SUBPROJECT_rev2
     #include "../../rev2/config.h"
+#endif
+#ifdef SUBPROJECT_rev2fliphalf
+	#include "../../rev2fliphalf/config.h"
 #endif