Adds the U-Boot loader for Banana pi, Cubieboard and Cubieboard2.

This version is patched so that:

• ELF and API features are enabled.
• The default environment is trimmed to just what's needed to boot.
• The saveenv command writes to the file u-boot.env on the FAT partition.
• The DTB file name is chosen based on the board model and passed to ubldr using the fdtfile env variable. ubldr loads the DTB from /boot/dtb/ on the FreeBSD partition.
• By default, it loads ELF ubldr from file ubldr on the FAT partition to address 0x42000000, and launches it.

For information about running FreeBSD on Allwinner SoCs, see
https://wiki.freebsd.org/FreeBSD/arm/Allwinner

Differential Revision: https://reviews.freebsd.org/D2874
Reviewed by: garga
Approved by: garga