Add a named CONFIG=cloud configuration, which enables console types
useful for obtaining output from virtual machines in public clouds
such as AWS EC2.

An image suitable for use in AWS EC2 can be built using

  make bin/ipxe.usb CONFIG=cloud EMBED=config/cloud/aws.ipxe

The embedded script will direct iPXE to download and execute the EC2
"user-data" file, which is always available to an EC2 VM via the URI
http://169.254.169.254/latest/user-data (regardless of the VPC
networking settings).  The boot can therefore be controlled by
modifying the per-instance user data, without having to modify the
boot disk image.

Console output can be obtained via syslog (with a syslog server
configured in the user-data script), via the AWS "System Log" (after
the instance has been stopped), or as a last resort from the log
partition on the boot disk.

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