config mode: warn if install is fresh but config hasn't been wiped

[Djfe]

add variable to uci that signifies: "this is a gluon installation"
if this variable is missing in uci but there is data in the user partition, suggest a wipe in config mode. (display as warning message)

We could also do something similar for sysupgrades between gluon communities and look at site_code.
Warn during sysupgrade if image doesn't contain the same site_code in the sysupgrade metadata as the current community has.
(Or ask for user confirmation instead; the config mode sysupgrade would need to be adjusted to support this)
Regarding the autoupdater: This could be valued in the same way as a minor compat upgrade => ignore if the update mirror offers the image and signed it.