Add multilocale Gecko/Gaia support

Define the B2G_LANGUAGE_PACK variable in the environment before a build
to select a preconfigured collection of locales.

Alternatively manually define the LOCALES_FILE, GAIA_DEFAULT_LOCALE, and
MOZ_CHROME_MULTILOCALE environment variables to create new combinations
without modifying this file, if desired.

Change-Id: I48e65c13f7f3b9fe2a2640150d492e6c2cf1e9b5