[question] package development workflow with custom setting

We have a package with a hardware dependency (size of available memory). We would like to build the package for different hardware using the package development workflow (https://docs.conan.io/en/latest/developing_packages/package_dev_flow.html). The build step is hardware-dependent.

To achieve this, we introduce a setting for the hardware, in `settings.yml`:
```
hardware: [r-2gb, r-4gb]
```
and a profile `myprofile`, where we specify compiler, arch, etc. and:
```
[settings]
.......
hardware=r-2gb
```
In the `conanfile.py` of our package we write
```
settings = "hardware"
```

Assuming the conanfile.py is in the local directory calling `conan create . --profile myprofile` is successful.

With the package development workflow and first step `conan source` we get
```
conan source . --source-folder tmp/source
ERROR: 'settings.hardware' value not defined
```

Is `conan source` not supported for custom settings? No profile can be specified as argument. The source is independent of the setting.

- [x] I've read the [CONTRIBUTING guide](https://github.com/conan-io/conan/blob/develop/.github/CONTRIBUTING.md).