Add initial support for building RPI2 images.

In release.sh, allow overriding buildenv_setup() before
the handoff to arm/release.sh.

Copy arm/RPI-B.conf -> arm/RPI2.conf, set UBOOT_PORT and
the correct KERNEL, and add the buildenv_setup() override
to install the sysutils/u-boot-rpi2 port/package.

Copy tools/arm/crochet-RPI-B.conf -> tools/arm/crochet-RPI2.conf,
and set the correct entries for the RaspberryPi2 board.

Thanks to: loos@
Sponsored by: The FreeBSD Foundation