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
Explicitly set XDEV_ARCH and TARGET_ARCH to 'armv6', which resolves
boot-time issues when building 10.0-RELEASE images.

Sponsored by:	The FreeBSD Foundation
Fix TARGET_ARCH and XDEV_ARCH for BEAGLEBONE, should be arm/armv6.

Sponsored by:	The FreeBSD Foundation
Remove EMBEDDED_WORLD_FLAGS from release.sh, and move the gcc(1)
build to arm/release.sh.

This prevents needing to build a separate chroot environment for
the arm/armv6 builds when it is not absolutely necessary.  This
is useful for situations where a single userland build is used
to populate more than one chroot.

Sponsored by:	The FreeBSD Foundation