can't use a gadget with a base when building a classic image with snap prepare-image

[Ian Johnson]

When trying to seed a classic image with `snap prepare-image`, if the gadget that is being included has a base, snapd refuses to seed the gadget snap unless a base is specified for the model:

```
$ sudo snap prepare-image --classic server-model.assert /tmp/img --snap edgex-pi-gadget/edgex-pi_18-1_armhf.snap
Fetching core
Copying "edgex-pi-gadget/edgex-pi_18-1_armhf.snap" (edgex-pi)
error: cannot use gadget snap because its base "core18" is different from model base ""
```

However adding a base to the model is not allowed by snap sign:

```
$ cat server-model.json | snap sign -k edgex-testing
error: cannot assemble assertion model: cannot specify a base with a classic model
```

Example server-model.json:

```
{
  "type": "model",
  "authority-id": "some-id",
  "brand-id": "some-id",
  "series": "16",
  "model": "edgex-rpi-delhi",
  "architecture": "arm64",
  "classic": "true",
  "base": "core18",
  "gadget": "edgex-pi",
  "required-snaps": [
    "edgexfoundry",
    "node-red",
    "edgex-device-grove"
  ],
  "timestamp": "2019-06-18T17:03:50+00:00"
}
```

This is problematic when trying to use the same gadget snap for both a classic image and a core image for the same physical hardware setup (to provide users both a ubuntu server image using these snaps and a ubuntu core image using these snaps). The gadget snap is necessary on a classic device so that there is slots for device access to use with strictly confined snaps.

[Samuele Pedroni (pedronis)]

this should have been fixed here:

https://github.com/snapcore/snapd/pull/7135

[Samuele Pedroni (pedronis)]

fix should be in snapd 2.42