[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.

assets/manifests/default

@@ -0,0 +1,5 @@
+kernel,kernel
+kernel,panic.serial
+user,drv.uefi_fb
+user,testapp
+user,srv.init