Bug 890065 - Add a flag for optionally disabling sources.xml

Change-Id: I93bac85dea9535a87569dda5cdcde9e1d45a5dba
Merge pull request #101 from dwilsoncaf/bug-890065

Bug 890065 - Add a flag for optionally disabling sources.xml r=mwu
Merge branch 'master' of https://github.com/mozilla-b2g/gonk-misc