kconfig: Use more collision resistant temporary filenames

kconfig: Use more collision resistant temporary filenames

kconfig creates reasonably safe filenames for its temporary files
except for two of them.

Change-Id: I6861f55ae2a5311e3fb7919333ce9af1e39ce78b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/408
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>