release: arm: Copy the dtb to the fat partition

When booting via EFI on arm we have no way to know the dtb file to load
and we always use the one provided from the bootloader.
This works in most case but :

U-Boot have some really old DTB for some boards, the sync from Linux isn't done automatically for all boards
Some boards (like TI BeagleBone series) use one u-boot for all the model and it doesn't embed the DTBs
Some boards (like IMX6 based ones), don't embed the DTB

We want u-boot to load and patch the DTB with the mac address or the display
node enabled or not.

Reviewed by: gjb, imp
Differential Revision: https://reviews.freebsd.org/D16596