loader.mk: Simplify a little bit

The elf loader needs to know how to reach into the gfx_fb code, so
simplify how we include files to find that stuff.

Sponsored by: Netflix
Reviewed by: manu, tsoome
Differential Revision: https://reviews.freebsd.org/D33376

(cherry picked from commit 2220b759d6b8cf912572fb22052edc5db4683dca)