generate_payload: Create extension update payloads

[pothos]

• download_payloads: Add helper script for update payload fetching

  The update payload fetching was a simple wget invocation until now but
  with OEM payloads and other future Flatcar extensions this gets more
  complicated.
  Add a script that finds out which extension payloads are available based
  on the Caddy JSON output.

• generate_payload: Create extension update payloads

  When a release has extension update payloads for OEM software these
  should be signed as well.
  Extend the generate_payload script to detect additional extension files
  and generate signed payloads for them.

How to use

cd /run/media/usb/sdk/src/scripts/data/
~/flatcar/flatcar-build-scripts/download_payloads alpha:1786.0.0 beta:1781.2.0
# continue inside as usual with ./generate_payload data/$ARCH/$VERSION keys/

Testing done

The above and

PROTOPATH=src/update_engine/ ./decode_payload ~/flatcar/scripts/sdk_container/src/third_party/coreos-overlay/coreos-base/coreos-au-key/files/official-v2.pub.pem /run/media/usb/sdk/src/scripts/data/test/3717.0.0/oem-qemu.gz /var/tmp/out

which was able to verify the signature.