MFC r262664, r262694:

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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@265069 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f