setup: remove obsolete windows and macos scripts

Don't delete v7+, it's needed for RpiZero2W

Don't delete v7+ modules, needed for Raspberry Pi Zero 2 W.
Add sntp, and use it to set the time before setup, to avoid problems
with certs being invalid.
Update for default cmdline changes.

Update prebuilt image

Add additional packages to the prebuilt image to save time during setup.
Increase root partition size to allow for easier updates after setup.
Update build instructions.

Update sample config

Condense overly verbose discussion of disk sizes, add a note
regarding the new 32GB minimum size warning that appeared in Tesla
software version 2020.48.10, and various minor corrections.

Update OneStepSetup.md

Stretch release is no more, so don't mention it in the setup doc.

Ignore fstrim failure during setup

Use automount to mount snapshots

This significantly improves startup time when many
snapshots are present.

Update files for prebuilt image

Update various files to work with the current pi-gen
Update rc.local to not save backup files when creating wpa_supplicant.conf
Update template wpa_supplicant.conf to include a country code
Remove stage_flash since it is no longer used
Minor wording change in teslausb_setup_variables.conf.sample

Update pi-gen-sources for Buster

Configure hostname in headless mode, add retry

setup-teslausb:
Always configure the host name, even in headless setup, and set the
specified name early on during setup, so the device is reachable by its
new host name during early setup.

prebuilt image sources:
Set the hostname when configuring wifi, so it's effective as soon as
wifi comes up.
Retry the setup-teslausb script download until it succeeds, since it
sometimes runs before the network is available and then fails.
Remove btrfs-tools and add xfsprogs to the release image packages.