The mozconfig detection logic has bitten us on many occasions in the
past. The following changes are made to tentatively improve the
situation:

- The API is modified such that autodetection of the mozconfig has
  to be a conscious decision made by the caller, and not triggered
  any time there is no mozconfig given, which could be a conscious
  decision of the opposite.

- mozinfo.json now stores the actual mozconfig (or lack thereof) used
  during configure.

--HG--
extra : rebase_source : c7a632afd414f25daf7bbe7e1a66c3736c26e039