genconfig.py: Print defconfig next to warnings

At present we sometimes see warnings of the form:

/tmp/tmpMA89kB:36: warning: overriding the value of CMD_SPL.
	Old value: "y", new value: "y".

This is not very useful as it does not show whch defconfig file it relates
to. Update the tool to show this.

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