pmb.aportgen: Ask whether a new device port is upstream or downstream

Essentially we want to place the generated APKBUILD files into the correct device/testing/ or device/downstream/ category, depending whether it's a port using upstream or downstream kernel.

An unfortunate side effect of this change is that when running "pmbootstrap aportgen device-foo-bar" we cannot know anymore what the user wants so we just place the port directly in device/ and not in any subdirectory. The user can move it manually where they desire.

Fixes #2556 (closed)

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Depends on !2549 (merged)

Depends on !2610 (merged)