Merge the following from ^/projects/release-arm64 to allow
building FreeBSD/arm64 VM images and memstick.img installation
medium:
r281786, r281788, r281792:
r281786:
Add support for building arm64/aarch64 virtual machine images.
r281788:
Copy amd64/make-memstick.sh to arm64/make-memstick.sh for aarch64 memory stick images. Although arm64 does not yet have USB support, the memstick image should be bootable with certain virtualization tools, such as qemu.
r281792:
Add a buildenv_setup() prototype, intended to be overridden as needed. For example, the arm64/aarch64 build needs devel/aarch64-binutils, so buildenv_setup() in the release.conf for this architecture handles the installation of the port before buildworld/buildkernel.
Sponsored by: The FreeBSD Foundation