implemented basic i{builder,maker} packaging for BFW

This is the "standard" way of doing things, where content
gets copied into a newly created instance, in this case
created by the bfwinstance script. If time allows a further
branch off this one will try to do it the non-copy way.

The main pieces of this are:

* A 'remotes' make target that uses t.b.instance info to
  put content in a pkgstore
* src/assets.recipe which defines the content to go in
  an assets bag
* bfwinstance which creates an instance using the pkgstore data
  and instance info
* adjusting layout.html to use the bag-based assets rather than
  remotes