[cloud] Add utility script to read iPXE output from INT13CON partition

[cloud] Add utility script to read iPXE output from INT13CON partition

Some AWS instance types still do not support serial console output or
screenshots.  For these instance types, the only viable way to extract
debugging information is to use the INT13 console (which is already
enabled via CONFIG=cloud for all AWS images).

Obtaining the INT13 console output can be very cumbersome, since there
is no direct way to read from an AWS volume.  The simplest current
approach is to stop the instance under test, detach its root volume,
and reattach the volume to a Linux instance in the same region.

Add a utility script aws-int13con to retrieve the INT13 console output
by creating a temporary snapshot, reading the first block from the
snapshot, and extracting the INT13 console partition content.

Signed-off-by: Michael Brown <mcb30@ipxe.org>