branches: 1.1.2;
Add installboot(8) support for evbarm (and, in the future, other evb*)
boards that use u-boot. A known board database lists boards and their
respective u-boot packages. u-boot packages are discovered at run-time
(in /usr/pkg/share/u-boot, by default). These packages contain board
database overlays that describe u-boot installation procedure that's
specific for that board.

Support this as a native tool and as a host tool. The native tool
will attempt to determine the running board type using OpenFirmware
calls. Host tool and native tool alike may also specify a board type
directly using the "-o board=xxx option" or have installboot(8) determine
the board type from a device tree blob using "-o dtb=/path/to/board.dtb".
A "-o media=xxx" option is provided for boards that have different u-boot
binaries and/or installation procedures for different media types (e.g.
SDMMC, eMMC, or USB).

This is trivial to extend to other evb* platforms that use u-boot, even if
they don't use FDT for autoconfiguration.