When building with a --enable-project that is neither js nor a
toolkit-based one (like browser or mobile/android), we don't want to be
building things that are specific to gecko and/or spidermonkey.
At the same time, this lifts the exception that js standalone doesn't
have an app.mozbuild included, and makes the moz.build parsers that
don't set a MOZ_BUILD_APP get the same information as they were through
toolkit.mozbuild.
We still keep mfbt, build and a few other DIRS set from the top-level,
because at the moment, there aren't really any --enable-project that
would benefit from those not being recursed.
