Binary Updates for aarch64

Our tool for building binary updates for base, hbsd-update-build, has now been taught how to cross-build. As of today, those who are experimenting with aarch64 devices like the Raspberry Pi 3 (RPI3) and Pine64 will be able to receive binary updates from HardenedBSD.

Since hbsd-update stages the binary update in /tmp, which is rather limited in size on most aarch64 dev boards, users will need to tell hbsd-update to use a different temporary directory:

# mkdir /root/tmp
# env TMPDIR=/root/tmp hbsd-update

Please note that aarch64 support is still a major work-in-progress both on FreeBSD's and HardenedBSD's side. Shawn Webb is working with both Ed Schouten and Ed Maste to resolve a few issues with lld 3.9.1. Once those issues are resolved, using aarch64 will become much easier for the average person.

Though this work is primarily targeted for aarch64 cross-building, it can be used with other architectures, like arm and i386 (i386 would need a follow-up commit). HardenedBSD plans only to provide binary updates for amd64 and aarch64 at this time.