[build] Move to yaml-based build config and manifest

Overall, I believe TOML to be a superior configuration format than YAML
in many situations, but it gets ugly quickly when nesting data
structures. The build configs were fine in TOML, but the manifest (and
my future plans for it) got unwieldy. I also did not want different
formats for each kind of configuration on top of also having a custom
DSL for interface definitions, so I've switched all the TOML to YAML.

Also of note is that this change actually adds structure to the manifest
file, which was little more than a CSV previously.

scripts/bonnibel/target.py

@@ -3,7 +3,7 @@ class Target(dict):
 
     @classmethod
     def load(cls, root, name, config=None):
-        import toml
+        from . import load_config
 
         if (name, config) in cls.__targets:
             return cls.__targets[(name, config)]
@@ -17,9 +17,9 @@ class Target(dict):
             basename += f"-{config}"
 
         while basename is not None:
-            filename = str(configs / (basename + ".toml"))
+            filename = str(configs / (basename + ".yaml"))
             depfiles.append(filename)
-            desc = toml.load(filename)
+            desc = load_config(filename)
             basename = desc.get("extends")
             dicts.append(desc.get("variables", dict()))