The default image group is already encoded in
/usr/share/flatcar/update.conf but it was written to
/etc/flatcar/update.conf as well. This can cause problems when the user
switches channels by forcing an update to a specific release from the
different channel (e.g., through the flatcar-update tool) as it leaves
the file under /etc/flatcar/update.conf out of sync with the new
channel version in /usr/share/flatcar/update.conf.
Since we don't really need to write a specific channel to /etc on new
images as we can rely on the value from /usr, we now leave any possible
overwriting of the value in /etc entirely to the user.
