This mostly moves a lists:reverse/1 which ensures that profile overlays
are run first but keeps the order of overlays otherwise.

This is so that some of them can be enforced
(such as warnings_as_errors) on relx's task like
generating a release or a release upgrade.

Relx config need new values at the end of the list and
profile additions prepended. So now in apply_profile in
the case it is a relx config we prepend the config list.
This does, however, mean that a profile release can not
'extend' a release defined in the default profile since
the one being extended would have its definition after
the profile release and thus relx would not be able to
use its values for extending.

fixes #872