weston-init: Add support for background configuration

Available knobs:

WESTON_INI_BACKGROUND_IMAGE - Full runtime path of the background image.
			      The base name of this path needs to be
                              available in FILEPATH (EXTRAFILESPATH).
WESTON_INI_BACKGROUND_COLOR - Background color - default to white.
WESTON_INI_BACKGROUND_TYPE  - Background type - defaults to centered.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>