release: Add arm_install_boot to install the commit boot bits

This reduce the per-board arm_install_uboot to just install u-boot.
While here remove the installation of rpi.dtb and rpi2.dtb as we load
them from the UFS partition via ubldr.

Reviewed by: gjb, imp (older version)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D16239