Allow projects to define their own build.

To create a new project, add a directory under `project` and pass the
project name to `make`:

    mkdir project/my_project
    # Describe the build for the project.
    PROJECT=my_project make

Projects can be maintained separately from the main repository.

Change-Id: I33a9d7ca801e2fb3dd9795ece44577b3b565e913
diff --git a/build/toolchain/platform.gni b/build/toolchain/platform.gni
index 7a1c8a9..4bcfdc3 100644
--- a/build/toolchain/platform.gni
+++ b/build/toolchain/platform.gni
@@ -14,9 +14,15 @@
 
 # Configuration of the build for the platform.
 declare_args() {
+  # The name of the platform.
+  plat_name = ""
+
+  # The architecture of the platform.
+  plat_arch = ""
+
   # The maximum number of CPUs available on the platform.
-  platform_max_cpus = 0
+  plat_max_cpus = 0
 
   # The maximum number of VMs required for the platform.
-  platform_max_vms = 0
+  plat_max_vms = 0
 }