[cloud] Add ability to retrieve Google Compute Engine metadata

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>

src/config/general.h

@@ -78,6 +78,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define HTTP_AUTH_BASIC		/* Basic authentication */
 #define HTTP_AUTH_DIGEST	/* Digest authentication */
 //#define HTTP_ENC_PEERDIST	/* PeerDist content encoding */
+//#define HTTP_HACK_GCE		/* Google Compute Engine hacks */
 
 /*
  * 802.11 cryptosystems and handshaking protocols