Upgrading the modem firmware of a Nordic Thingy:91 over MCUBoot serial recovery

The traits mcuBoot and modem indicate that the Nordic Thingy:91™ supports modem upgrades over MCUBoot serial recovery.

1. List the devices connected to the system by running the following:

   nrfutil device list --traits mcuboot,modem

2. Put the Nordic Thingy:91 in application serial recovery mode by holding in the SW3 button while powering it off and on.
3. Upgrade the modem firmware by running:

   nrfutil device program --firmware mfw_nrf9160_1.3.3.zip --serial-number THINGY91_xxxxxxxxxxxx