In order to determine if we need to re-run configure, we write
a JSON file representing the evaluated mozconfig. If this JSON
file changes, configure (and config.status for that matter) is
out of data and it is re-executed.

This commit moves the generation of that JSON file to Python.

MozReview-Commit-ID: 636rpSY7gOm

--HG--
extra : rebase_source : ee1defd74decfd64ffb66a45b053dada58de04fb