pine64: cleanups

starting by adjusting partitions. this will likely need several commits.

system-image is going to be a much bigger task. it has been further pushed to be more android-centric, which has basically blocked us from having it in a usable state on mainline on focal right now. we will need to completely reconsider how writable paths are configured. right now the system keeps itself rw, and adding a userdata partition was only causing problems with the devices.

for now, we are renaming our main partition to "system" to match at the very least.

additionally, mainline variants should not be told to install android symlinks. things like this is exactly why chroot hooks have the VARIANT flag.

also, pinetab2's wifi is in an experimental state, and probably will be for a while, therefore it is disabled by default. before, we were telling users to just echo these modules by hand to enable them if they so choose, but this now allows for a safer way for users to do so, with sudo systemctl start pinetab2-wifi-{enable,disable}. These are kept not persistent and so cannot be systemctl enable'd.

and keep a full safety partition for the boot loaders, if installed.