[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/cloud/gce.ipxe

@@ -0,0 +1,7 @@
+#!ipxe
+
+echo Google Compute Engine - iPXE boot via metadata
+ifstat ||
+dhcp ||
+route ||
+chain -ar http://metadata.google.internal/computeMetadata/v1/instance/attributes/ipxeboot

src/config/cloud/general.h

@@ -0,0 +1,4 @@
+/* Allow retrieval of metadata (such as an iPXE boot script) from
+ * Google Compute Engine metadata server.
+ */
+#define HTTP_HACK_GCE