Merge the following from ^/projects/release-arm64 to allow

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