x86: Add ifwitool for Intel Integrated Firmware Image

Some Intel SoCs from about 2016 boot using an internal microcontroller via
an 'IFWI' image. This is a special format which can hold firmware images.
In U-Boot's case it holds u-boot-tpl.bin.

Add this tool, taken from coreboot, so that we can build bootable images
on apollolake SoCs.

This tool itself has no tests. Some amount of coverage will be provided by
the binman tests that use it, so enable building the tool on sandbox.

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