Fix coreboot makefiles not to produce half baked output.

Fix coreboot makefiles not to produce half baked output.

There were cases where output file was generated and modified within
a recipe. If make was interrupted, it could exit with an output file
that appears as up-to-date, but was generated with incomplete recipe.

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: I25c8ee23577a460eace196fd28c23cc67aa72a9a
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/830
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>