loader: Move to using linker sets to bring in optional bits

The graphics stuff is optional. When it is pulled into the system, we
use a linker set to initialize the lua bindings for it now.

Sponsored by: Netflix
Reviewed by: kevans, jhb
Differential Revision: https://reviews.freebsd.org/D43906

(cherry picked from commit 0921a771da8a9117edf26352a8a047bacbfcee45)