weston-init: Add support for a dynamic .ini configuration

This creates a mechanism to generate a dynamic weston.ini configuration
based on build variables.

The entire mechanism is enabled using the WESTON_DYNAMIC_INI variable
(enabled when set to 1).

The default path for the weston.ini configuration is set in
WESTON_INI_PATH as "etc/xdg/weston/weston.ini".

WESTON_INI_NO_TOOLBAR, when set to 1, disables the weston toolbar.

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