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>