clean up Makefile

The change that prompted this branch was the merger of the profile and prosthesis targets, since they depended on each other, and thus it was misleading to have both (and users who did `make profile` would get a broken profile that couldn't be used with the addon because the prosthesis addon wouldn't be installed in B2G).

But along the way I identified a bunch of other things to improve and have implemented them. The commit log should explain each change.