Add "official" and "unofficial" build configs

[joneschrisg]

CC @mvines @michaelwu

Currently all b2g builds are configured to automatically update gaia and gecko. That's cool, except when you want to test with a local version, which is 100% of the time for developers.

We should add new targets -
config--official
config--unofficial

and have config-* default to unofficial. This will require all developers to reconfigure, so it's probably best to save this change for the Great Repo Switch-over.

[mvines]

How about we leverage the existing eng vs user/userdebug build flavors? |make config-xyz| seems likely to go away when TGRS occurs, in fact it already has for me:
$ . build/envsetup && (lunch xyz-eng || lunch xyz-userdebug )

[joneschrisg]

Works for me. I don't really care about the names.

Phone