For some unspecified "security" reason, the Google Compute Engine
metadata server will refuse any requests that do not include the
non-standard HTTP header "Metadata-Flavor: Google".

Attempt to autodetect such requests (by comparing the hostname against
"metadata.google.internal"), and add the "Metadata-Flavor: Google"
header if applicable.

Enable this feature in the CONFIG=cloud build, and include a sample
embedded script allowing iPXE to boot from a script configured as
metadata via e.g.

  # Create shared boot image
  make bin/ipxe.usb CONFIG=cloud EMBED=config/cloud/gce.ipxe

  # Configure per-instance boot script
  gcloud compute instances add-metadata <instance> \
         --metadata-from-file ipxeboot=boot.ipxe

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

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>