MFC r336598-r336600, r336721

r336598:
release: Uniform release arm image size

Create a 50m fat16 for all boards.

Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D16238

r336599:
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)
Differential Revision: https://reviews.freebsd.org/D16239

r336600:
release: Add rpi firmware dtb and mmc overlays for RPI-B and RPI2

This is needed with new u-boot that uses the rpi-firmware dtbs.

Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D16240

r336721:
release: Add OL_DIR forgot in r336600

Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D16439

Approved by: gjb (for insta-mfc of r336721)