x86: Makefile: Drop explicit targets built by binman

On x86 various files that need to be created by binman. It does not make
sense to enumerate these in the Makefile. They are described in the
configuration (devicetree) for each board and we can simply run binman
(always) to generate them.

Update the Makefile to have a separate, final step which runs binman,
once all input dependencies are present.

This avoid sprinkling the Makefile with arch-specific code.

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