Some modems (e.g. the XBee Cellular LTE Cat 1) have a long initilization
process and are not available when ModemManager tries to detect them
on boot.
For those modems schedule a rescan a bit later after NetworkManager has
been launched, so they are detected properly.
https://jira.digi.com/browse/DEL-4661
Signed-off-by: Javier Viguera <javier.viguera@digi.com>