[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/srv.init/init.module

@@ -1,7 +1,7 @@
 # vim: ft=python
 
 init = module("srv.init",
-    targets = [ "init" ],
+    target = "init",
     deps = [ "libc", "elf", "bootproto", "zstd", "acpi", "pci" ],
     static = True,
     description = "Init server",