Build - Add support for specifying build variant in brunch/breakfast

This little modification allows specifying build variant in brunch/
breakfast commands. For example we can use "brunch i9300 user" to build
user variant instead of default userdebug.

When no extra argument is given, userdebug is default.

Change-Id: I244efcd9ddac2355cc96531f587399c19fa6cf52
Signed-off-by: STELIX <ssspinni@gmail.com>
Build - Clean up use of vendor/extra

* extra_config.mk is not needed anymore
* Include BoardConfigExtra.mk if available

Change-Id: I3581771ae6ea3c87de6c389808d97983262efe63
Signed-off-by: STELIX <ssspinni@gmail.com>