Create a custom /boot/defaults/loader.conf for ARM. This differs from the
standard file in the following ways:

• modules_path includes /boot/dtb
• It doesn't contain 533 lines, of which 500 are either commented out, empty, or something_which_doesnt_work_on_arm_anyway=NO

The standard defaults file takes 40+ seconds to process on an arm beaglebone
board. This one takes just a couple seconds.

This gets installed instead of the original because of the .PATH magic in
the makefile.