[build] Move configs/ to assets/build/

I was sick of `configs` colliding with `configure`, and these are
(mostly) static asset files anyway, they should live under assets/.

scripts/bonnibel/project.py

@@ -30,7 +30,7 @@ class Project:
             build.variable("source_root", root)
             build.newline()
 
-            build.include(root / "configs" / "rules.ninja")
+            build.include(root / "assets/build/rules.ninja")
             build.newline()
 
             build.variable("version_major", self.version.major)

scripts/bonnibel/target.py

@@ -8,7 +8,7 @@ class Target(dict):
         if (name, config) in cls.__targets:
             return cls.__targets[(name, config)]
 
-        configs = root / "configs"
+        configs = root / "assets/build"
 
         dicts = []
         depfiles = []