Fixes compilation by 'guix pull' where Guile-{RSVG,Cairo} are missing.
Reported by Efraim Flashner.
* gnu/build/svg.scm: Use 'module-autoload!' rather than 'module-use!'.
Based on a suggestion by Andy Wingo at
<https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00753.html>.
* gnu/build/svg.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* gnu/system/grub.scm (svg->png): Add #:width and #:height. Rewrite to
use (gnu build svg).
(resize-image): Remove.
(grub-background-image): Adjust accordingly.