kbuild: minor scripts/decodecode update

Remove the tmp file when exiting.  Noticed by Arjan van de Ven.
Catch mktemp failure and exit with message.
Trap kill or other signals and exit cleanly.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>