Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Initial support of loader(8) for ARM machines running U-Boot.

This uses the common U-Boot support lib (sys/boot/uboot, already used on
FreeBSD/powerpc), and assumes the underlying firmware has the modern API for
stand-alone apps enabled in the config (CONFIG_API).

Only netbooting is supported at the moment.

Obtained from: Marvell, Semihalf

Don't descend into at91 until we sort out the boot loader issues more generally

Build at91

Add a dummy makefile to keep build happy