Several popular public cloud providers do not provide any sensible
mechanism for obtaining debug output from an OS which is failing to
boot.  For example, Amazon EC2 provides the "Get System Log" facility,
which occasionally deigns to report a random subset of the characters
emitted via the VM's serial port, but usually returns only a blank
screen.  (Amazingly, this is still superior to the debugging
facilities provided by Azure.)

Work around these shortcomings by adding a console type which sends
output to a magically detected raw disk partition, and including such
a partition within any iPXE .usb-format image.

To use this facility:

 - build an iPXE .usb image with CONSOLE_INT13 enabled

 - boot the cloud VM from this image

 - after the boot fails, attach the VM's boot disk to a second VM

 - from this second VM, use "less -f -R /dev/sdb3" (or similar) to
   view the iPXE output.

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