what-jenkins-does: kill build results early

This reduces disk use and simplifies using abuild on
a ramdisk.

Change-Id: I3fb8d273dcbb5008fa9cfaa9465a59e3bbcb974b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6835
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>