[coreboot] [PATCH]Add "update image" feature

[Patrick Georgi]

Hi,

attached image adds image updating support. When selecting it, it
expects a coreboot.rom to be available, and adds the files to it.

It has no idea how to replace files, it merely adds them. It only works
with Tinybootblock and the bootblock is immutable.

The "clean" rules were changed to allow "make clean-for-update", which
removes everything but coreboot.rom

Signed-off-by: Patrick Georgi