Make better use of toolchains in the build.

This allows building for different platforms without having to retarget
the build. The configuration is also better checked by the assertions in
the toolchain templates.

A future change will redirect the root build rule to allow BSPs to
fully control their own build and configuration.

Change-Id: Iaae725d3bd000bc0ce7b9ef0f8f083350a73bd16
diff --git a/build/toolchain/platform.gni b/build/toolchain/platform.gni
new file mode 100644
index 0000000..7a1c8a9
--- /dev/null
+++ b/build/toolchain/platform.gni
@@ -0,0 +1,22 @@
+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Configuration of the build for the platform.
+declare_args() {
+  # The maximum number of CPUs available on the platform.
+  platform_max_cpus = 0
+
+  # The maximum number of VMs required for the platform.
+  platform_max_vms = 0
+}