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.

Merge ^/projects/release-arm-redux into ^/head.

Of note:

- This commit adds native FreeBSD/arm release build support without
  requiring out-of-tree utilities.

- Part of this merge removes the WANDBOARD-{SOLO,DUAL,QUAD} kernel
  configuration files, for which the IMX6 kernel configuration file
  should be used instead.

- The resulting images have a 'freebsd' user (password 'freebsd'),
  to allow ssh(1) access when console access is not available (VGA
  or serial).  The default 'root' user password is set to 'root'.

- The /etc/ttys file for arm images now enable both ttyv0 and ttyu0
  by default.