sconfig: more careful string resource handling

When parsing a string to numbers, we don't need to copy it.
And when creating strings, we should eventually free them.

Change-Id: I9023fef6e97a1830bc68502be32e79879c1617d4
Found-By: Coverity Scan
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6484
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>