Add platform mappings to make --cpu work again

Without using --cpu, changing the platform was resulting in a full
rebuild since the cache was per cpu, not per platform.  I'm sure there's
a way to handle this, but I know that adding a platform mapping will
make the caches work again.

Change-Id: I5b9a64e0cc27465b04e5991c2c1b3d9c6dea41e6
diff --git a/platform_mappings b/platform_mappings
new file mode 100644
index 0000000..04265dd
--- /dev/null
+++ b/platform_mappings
@@ -0,0 +1,26 @@
+# https://docs.bazel.build/versions/master/platforms-intro.html#platform-mappings
+platforms:
+  //tools/platforms:linux_x86
+    --cpu=k8
+
+  //tools/platforms:linux_roborio
+    --cpu=roborio
+
+  //tools/platforms:linux_armhf
+    --cpu=armhf-debian
+
+  //tools/platforms:cortex_m4f
+    --cpu=cortex-m4f
+
+flags:
+  --cpu=k8
+    //tools/platforms:linux_x86
+
+  --cpu=roborio
+    //tools/platforms:linux_roborio
+
+  --cpu=armhf-debian
+    //tools/platforms:linux_armhf
+
+  --cpu=cortex-m4f
+    //tools/platforms:cortex_m4f