Fix coreboot makefiles not to produce half baked output.

Fix coreboot makefiles not to produce half baked output.

It looks like the cbfstool utility generates the output file even when
it fails to generate it properly. This causes make, if started second
time in a row, after cbfstool failure, to continue beyond the point of
failure (as the corrupted output file is present in the output tree,
the second make invocation presumes that it is valid, as it is newer
than the dependencies).

The output file should be created only when successful, in an atomic
operation. There could be other places in the make system which
require a similar fix, this needs to be investigated further.

Change-Id: I7c17f033ee5937eb712b1a594122430cee5c9146
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: http://review.coreboot.org/750
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)