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 layouts
and dictionaries we need for a particular build.

Dictionaries live in gaia/keyboard/dictionaries and layouts are
defined in gaia/keyboard/layouts. Files are copied into this app, but
are not checked into github in this app.

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 actual configuration code is in build/keyboard-config.js. The
functions defined by that file are used by build/application-data.js
and build/webapp-manifests.js

