If the `--flash-contents` switch is specified more than once, it will
result in a memory leak. Therefore, allow this option only once.

Change-Id: I530933c9a6431580fe4645396bb363939472a80a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/39972


Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>