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.
MFC r262664, r262694:

  Prevent fdt data loaded from a file from overwriting the kernel environment,
  by having uboot_autoload() do the fdt setup (which may load a file) rather
  than waiting until we're actually in the process of launching the kernel.

  As part of making this happen...
   - Define LOADER_FDT_SUPPORT on the uboot/lib compile command line when
     MK_FDT is set.
   - Make fdt_setup_fdtb() public.
   - Declare public fdt_whatever() functions in a header instead of using
     scattered extern decls in .c files.
MFC r262666: exit with code 0xbadab1 if the u-boot API support is missing.