The keyboard app uses large dictionary files for autocorrection. If we
include all the dictionaries we have, our builds become too large. So
the keyboard is configured at build time to include only the IMEs,
layouts, and dictionaries we need for a particular build.

Dictionaries files currently only apply for the latin IME and live in
js/imes/latin/dictionaries, along with the source files (in XML) in
Android format. There is a Makefile present to convert the source files
into Firefox OS dictionaries.

At build time we only package the IMEs and dictionaries required by the
specified keyboard layouts.

We also configure the manifest.webapp file at build time, to add entry
points for each layout.

Configuration is done with the GAIA_KEYBOARD_LAYOUTS environment
variable, and there is a default value defined in the Makefile.

The build happens in gaia/build_stage.
The actual configuration code is in build/keyboard-config.js.
