binman: Support updating the dtb in an ELF file

WIth EFI we must embed the devicetree in an ELF image so that it is loaded
as part of the executable file. We want it to include the binman
definition in there also, which in some cases cannot be created until the
ELF (u-boot) is built. Add an option to binman to support writing the
updated dtb to the ELF file u-boot.out

This is useful with the EFI app, which is always packaged as an ELF file.

Signed-off-by: Simon Glass <sjg@chromium.org>