Bundle Yamale schema

TIL! Using the existence of obscure member sys._MEIPASS, I can detect if
we're running from a bundled PyInstaller binary, versus running from Python
directly.

Add schema.yaml to the datas of the PyInstaller spec. Then, if the
-/--schema flag is given, use it, and if it's not given, default to the
built-in one if we're running from a bundled binary, or fall-through to
./schema.yaml in other cases.

This avoids the need for config/schema.py as a carbon-copy of the schema,
slick!

vppcfg.spec

@@ -4,10 +4,12 @@
 block_cipher = None
 
 
+added_files = [ ( 'schema.yaml', '.') ]
+
 a = Analysis(['vppcfg'],
              pathex=['/home/pim/src/vppcfg'],
              binaries=[],
-             datas=[],
+             datas=added_files,
              hiddenimports=[],
              hookspath=[],
              hooksconfig={},