[build] Move to python build scripts per module

This change moves Bonnibel from a separate project into the jsix tree,
and alters the project configuration to be jsix-specific. (I stopped
using bonnibel for any other projects, so it's far easier to make it a
custom generator for jsix.) The build system now also uses actual python
code in `*.module` files to configure modules instead of TOML files.
Target configs (boot, kernel-mode, user-mode) now moved to separate TOML
files under `configs/` and can inherit from one another.

src/user/testapp/testapp.module

@@ -0,0 +1,10 @@
+# vim: ft=python
+
+module("testapp",
+    targets = [ "user" ],
+    deps = [ "libc" ],
+    sources = [
+        "io.cpp",
+        "main.cpp",
+        "serial.cpp",
+    ])