d1: define subtarget specifically

Historically it's possible to leave the `SUBTARGETS` undefined and
automatically fallback to a "generic" subtarget. This however breaks
various downstream scripts which may have expectations around filenames:

While some targets with an explicit generic subtarget contain `generic`
in the filenames of artifacts, implicit "subtargets" don't.

Right now this breaks the CI[1], possibly also scripts using the ImageBuilders.

Do to the D1 target what's done to other target, explicitly define the
"generic" subtarget.

[1]: https://github.com/openwrt/openwrt/actions/runs/8592821105/job/23548273630

Signed-off-by: Paul Spooren <mail@aparcar.org>

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