[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/config_http.c

@@ -43,3 +43,6 @@ REQUIRE_OBJECT ( httpdigest );
 #ifdef HTTP_ENC_PEERDIST
 REQUIRE_OBJECT ( peerdist );
 #endif
+#ifdef HTTP_HACK_GCE
+REQUIRE_OBJECT ( httpgce );
+#endif