fastboot: fix issues and improve output of bootgraph.pl

David Sanders reported some issues with bootgraph.pl's display
of his sytems bootup; this commit fixes these by scaling the graph
not from 0 - end time but from the first initcall to the end time;
the minimum display size etc also now need to scale with this, as does
the axis display.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>