Author: Bas Brands
MDL-69371 theme_boost: allow configuring a login background image
http://git.moodle.org/gw?p=moodle.git;a=commit;h=7c2e6d07af5991b79597baa4662114d6de12cb46
http://github.com/moodle/moodle/commit/7c2e6d07af5991b79597baa4662114d6de12cb46
+ 4.0 en [loginbackgroundimage,theme_boost]
+ 4.0 en [loginbackgroundimage_desc,theme_boost]