buffet: Rework buffet configuration

Manufacturer supplied fields will come from the buffet configuration
file with good defaults provided by buffet itself.  This includes
a few default values for user supplied fields.

User supplied and other per device instance state will be stored in
a state file as before.

Keeping these fields separate greatly simplifies reasoning about
adding configuration settings to buffet.

BUG=brillo:658
TEST=unittests, tendo_experimental passes
CQ-DEPEND=CL:262292

Change-Id: Ib74721b9c99d11c189042aa78cc43a076072de32
Reviewed-on: https://chromium-review.googlesource.com/262296
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Anton Muhin <antonm@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
diff --git a/buffet/main.cc b/buffet/main.cc
index 6893723..58d70d3 100644
--- a/buffet/main.cc
+++ b/buffet/main.cc
@@ -42,6 +42,7 @@
   }
 
  private:
+  BuffetConfig config_;
   std::unique_ptr<buffet::Manager> manager_;
   const base::FilePath config_path_;
   const base::FilePath state_path_;