binman: Support packaging U-Boot for scenarios like OF_BOARD or OF_PRIOR_STAGE

For scenarios like OF_BOARD or OF_PRIOR_STAGE, no device tree blob is
provided in the U-Boot build phase hence the binman node information
is not available. In order to support such use case, a new Kconfig
option BINMAN_STANDALONE_FDT is introduced, to tell the build system
that a device tree blob containing binman node is explicitly required
when using binman to package U-Boot.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>