binman: Add support for building x86 ROMs

The structure of x86 ROMs is pretty complex. There are various binary blobs
to place in the image. Microcode requires special handling so that it is
available to very early code and can be used without any memory whatsoever.

Add support for the various entry types that are currently needed, along
with some tests.

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