build: General build refactoring

Add vars.mk for variable sharing across makefiles.
Stub and rename example YAML file and template.
  Content to be re-introduced as support is added.
Move generated C++ to a header file.
Add input YAML path option to configure.

Change-Id: Ia06e392e8b120b1e1d8614046064fa432a6f9e18
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/.gitignore b/.gitignore
index 0bc0878..d1d026c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,3 +24,4 @@
 missing
 stamp-h1
 phosphor-dbus-monitor
+generated.hpp