Merge ^/projects/release-embedded into ^/head.

Merge ^/projects/release-embedded into ^/head.

After several months of testing and fixing (and breaking)
various parts of release/release.sh changes, it is now
possible to build FreeBSD/arm images as part of the release
process.

When EMBEDDEDBUILD is set in the release.conf file, release.sh
will create the build environment, then run a separate script
in release/${XDEV}/release.sh [1].  Currently, only arm is
supported.

The release/${XDEV}/release.sh configures the build environment
specific for the target image, such as installing gcc(1),
installing additional third-party software from the ports tree,
and fetching external sources.

Once the build environment is set up, release/${XDEV}/release.sh
runs Crochet, written by Tim Kientzle, which builds the userland
and kernel, and creates an image that can be written to an SD
card with dd(1).  Many thanks to Tim for his work on Crochet.

Sample configurations for FreeBSD/arm boards are in the
release/arm/ directory, and Crochet configuration files for each
board are located in release/tools/arm/.  Supported boards at this
time are: BEAGLEBONE, PANDABOARD, RPI-B, and WANDBOARD-QUAD.

Adding support for additional boards will continue in the
projects/release-embedded/ branch, and incrementally merged back
to head/.

Many thanks to the FreeBSD Foundation for the support and
sponsorship of this project.

[1] XDEV is used in order to keep the various configurations
    organized by architecture, but since TARGET and TARGET_ARCH
    are used to build the chroot, the values of those variables
    cannot be used.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation