Bug 935059 - Add partial and full FOTA targets

Adds two targets: gecko-update-fota and gecko-update-fota-full. The
later will produce a classical FOTA update of the whole /system
partition while the first one will provide a configurable scope-limited
update: by default, only /system/b2g. This directory can be altered by
setting the B2G_FOTA_DIRS environment variable, e.g. running:
$ B2G_FOTA_DIRS="system/b2g system/fonts" ./build.sh gecko-update-fota
Will produce a FOTA package only containing system/fonts and system/b2g.