Select one unity_config.h directory

Fix Issue #816 by appending only the first directory found
containing a unity_config.h to the include path

Author: jmwanderer

platformio-build.py

@@ -20,5 +20,7 @@
         p = projenv.subst(p)
         if os.path.isfile(os.path.join(p, "unity_config.h")):
             env.Prepend(CPPPATH=[p])
+            # Stop at the first unity_config.h found to include.
+            break
 except:
     pass