MFC r262340, r262345, r262347, find and load an appropriate dtb file.

MFC r262340, r262345, r262347, find and load an appropriate dtb file.

The search order for a usable dtb in fdt_setup_fdtp() is now

 - A dtb loaded with an explicit "load -t dtb" command.
 - A dtb already loaded into memory somehow[*] and pointed to by fdt_to_load.
 - A dtb in the memory pointed to by the u-boot env vars fdtaddr or fdt_addr.
 - A file named by the u-boot env vars fdtfile or fdt_file.
 - A static dtb compiled into the kernel.

 * Presumably by some arch-specific command or code.

git-svn-id: svn://svn.freebsd.org/base/stable/10@265068 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f