Switch to a new cortex-m4 compiler

This one is newer, so it supports a newer C++ standard. It's also
hermetic.

Also add a variant for building for some Kinetis-K chips with less RAM
that are used on fet12v2.

Change-Id: I9e50b6aae498e0c35acfedb846b3ada619a0e630
diff --git a/motors/fet12/fet12.cc b/motors/fet12/fet12.cc
index 6fedeab..f008379 100644
--- a/motors/fet12/fet12.cc
+++ b/motors/fet12/fet12.cc
@@ -306,6 +306,8 @@
   delay(1000);
 #if 0
   ZeroMotor();
+#else
+  (void)ZeroMotor;
 #endif
 
   printf("Zeroed motor!\n");