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/simple_receiver.cc b/motors/simple_receiver.cc
index f8b38f0..3b174b0 100644
--- a/motors/simple_receiver.cc
+++ b/motors/simple_receiver.cc
@@ -188,7 +188,8 @@
   can_send(id, data, sizeof(data), 2 + vesc_id);
 }
 
-void DoVescTest() {
+// TODO(Brian): Move these two test functions somewhere else.
+__attribute__((unused)) void DoVescTest() {
   uint32_t time = micros();
   while (true) {
     for (int i = 0; i < 6; ++i) {
@@ -212,7 +213,7 @@
   }
 }
 
-void DoReceiverTest2() {
+__attribute__((unused)) void DoReceiverTest2() {
   static constexpr float kMaxRpm = 10000.0f;
   while (true) {
     const bool flip = convert_input_width(2) > 0;