bootgraph: make the bootgraph script show async waiting time

It is useful for diagnosing boot performance to see where async function
calls are waiting on serialization...  this patch adds this
functionality to the bootgraph.pl script.

The waiting time is shown as a half transparent, gray bar through the
block that is waiting.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>