[build] first pass at multiarch support

Changing bonnibel to respect the --arch flag to configure. This requires
some reworking of modules, mostly in the addition of the ModuleList
class instead of just a dict of modules.

src/user/testapp/testapp.module

@@ -1,7 +1,7 @@
 # vim: ft=python
 
 module("testapp",
-    targets = [ "user" ],
+    target = "user",
     deps = [ "libc" ],
     description = "Testbed app",
     sources = [