Use /boot/loader.conf.d/ for GRE module loading
Summary:
Move the GRE kernel module configuration on FreeBSD from a direct modification of /boot/loader.conf to a dedicated managed file
in /boot/loader.conf.d/gre.conf.
Reviewers: dereckson, Duranzed
Reviewed By: dereckson
Subscribers: Duranzed, dereckson
Differential Revision: https://devcentral.nasqueron.org/D4007