ifdtool: Provide bounds on string parser

While the result will not be pretty (ie. ifdtool will
mis-parse string components longer than 255 characters),
at least it doesn't overflow stack variables anymore.

Change-Id: I263c5cf823a2d8a863dcece7c4ee0b26475f9fc4
Found-by: Coverity Scan
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6562
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)