internal: Fix board vendor and model memory leaks

The board vendor and model are sometimes specified as arguments during
an internal flash, so make sure they are freed at the end of
initialization.

Change-Id: I9f43708f3b075896be67acec114bc6f390f8c6ca
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1230664, 1230665
Reviewed-on: https://review.coreboot.org/c/flashrom/+/34846


Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>