Downstream kernel specific package
Generate a template
Usually you would generate both a new device specific package and a new vendor kernel package by running
pmbootstrap init and typing in a new device name. In case you should only have a device package, but not a kernel package, you can regenerate the latter with
pmbootstrap aportgen linux-changeme-changeme (you guessed it, changeme needs to be adjusted).
The template gets improved every now and then to make porting easier, so don't be surprised when you are looking at older ports and their APKBUILDs look a bit more complicated than what gets generated from the latest
pmbootstrap version. Essentially they are doing the same thing.