build system: move defconfig creation into a cbfs-files filter

That allows this special case to become a normal cbfs-files instance,
too.

Change-Id: I896ffebe4cec64c9c11605b4f09c7790e5419928
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/12539
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
1 file changed