bootgraph.pl: relax timing information requirements

This patch removes the assumption of the bootgraph.pl script that the
timing information reported by PRINTK_TIME will contain at least one
entry with a time of less than 100 seconds.

Not all boards correctly reset the system timer and in many cases the
inital times reported by PRINTK_TIME is high. When this occurs the
bootchart.pl script fails to give any useful output.

This patch sets the $firsttime variable to the largest value expected
by PRINTK_TIME

Signed-off-by: Andrew Murray <amurray@mpc-data.co.uk>
Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>