Copy resulting images to top level /R directory of the chroot
to match behavior of 'make install' target of src/release/.
Generate sha256 and md5 checksums while here.

Sponsored by:	The FreeBSD Foundation
Add initial support for the PANDABOARD.

Sponsored by:	The FreeBSD Foundation
Compress arm/armv6 images before generating checksums to reduce
image size for download.

Sponsored by:	The FreeBSD Foundation
Include 'TARGET-TARGET_ARCH' in the resulting image name similar
to 'make -V OSRELEASE' output.

This probably should use 'make -V OSRELEASE' directly, but for
now, this keeps the line length reasonable for readability.

Sponsored by:	The FreeBSD Foundation