buffet: load standard GCD command definitions in CommandManager

Changed CommandManager to load the base command definitions
and use it as a base schema for loading device-specific commands
and make sure device vendors are not redefining standard commands
in breaking manner.

BUG=chromium:374861
TEST=USE=buffet P2_TEST_FILTER="buffet::*" FEATURES=test emerge-link platform2

Change-Id: I5f2d5500bc90ef918a8a6df1242bdd1fe3b78615
Reviewed-on: https://chromium-review.googlesource.com/209175
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>

buffet/commands/schema_constants.h

diff --git a/buffet/commands/schema_constants.h b/buffet/commands/schema_constants.h
index 6a508e0..903228e 100644
--- a/buffet/commands/schema_constants.h
+++ b/buffet/commands/schema_constants.h
@@ -24,6 +24,7 @@
 extern const char kUnknownProperty[];
 extern const char kInvalidObjectSchema[];
 extern const char kDuplicateCommandDef[];
+extern const char kInvalidCommandName[];
 }  // namespace commands
 }  // namespace errors