x86: Add ifdtool for working with Intel Flash Descriptor ROM images

Newer Intel chips require a Management Engine which requires a particular
format for the SPI flash that contains the boot loader. Add a tool that
supports creating and modifying these ROM images.

This tool is from Chrome OS but has been cleaned up to use U-Boot style
and to add comments. A few features have been added also.

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