Add initial support for building arm BEAGLEBONE images.

• In arm/release.sh:
  • Move crochet work directory creation to occur earlier, since its parent directory is needed to store a fetched u-boot version.
  • Add a before_build() function as a quick hack to run board-specific (kernel configuration specific, really) commands, if any.
  • For the BEAGLEBONE kernel, a specific version of u-boot source is needed. Fetch the source in before_build() for this case. Unfortunately, there are no checksums available (that I can find) for these sources. For a quick solution, add the hard-coded sha256 of the tarball, and fetch from my public_html/ directory. A more permanent solution for this needs to be found.

Add initial release.sh and crochet configuration files for the
BEAGLEBONE build.

Sponsored by: The FreeBSD Foundation