board_r: Relocate OF_EMBED if NEEDS_MANUAL_RELOC only

When the embedded device tree is pointed to by the __dtb_dt_*begin
symbols, it seems to be covered by the early relocation code and doesn't
need to be manually patched.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>