device_credentials_installer: --id-imei option uses AT command string instead of IMEI for device ID

[Mattheish]

Description:
The device_credentials_installer tool, when used with the --id-imei option, incorrectly uses an AT command string as the device ID instead of the modem's 15-digit IMEI. This causes issues during the nRF Cloud onboarding process, as the cloud service cannot correctly interpret the AT command string as a device ID.

Steps to Reproduce:

1. Flash a device with the modem shell sample application.
2. Run device_credentials_installer with --id-imei
   example:

device_credentials_installer --dv 3650 --csv "path/to/onboard.csv" -a -d --devinfo "path/to/devinfo.csv" --devinfo-append --rtt --verify --id-imei --id-str "RD" --replace --ca CA_FILE --ca-key CA_KEY_FILE

3. Check logs or onboard.csv for result

Expected Behavior:
The device ID should be the 15-digit IMEI of the modem

Actual Behavior:
The device ID is set to an AT command string

Impact:
This prevents successful device onboarding to nRF Cloud, as the device ID is not in the expected format.

Environment:
nrfcloud-utils version: 2.0.0

[Mattheish]

Actually this might be a broader issue, after onboarding the device without --id-imei it becomes apparent that under the shadow in the modem firmware there is also AT+ command.

modemFirmware: "AT+CGMRmfw_nrf91x1_2.0.2"

[maxd-nordic]

Hi! Sorry for the delay. You are right. There is a problem with the RTT backend. I will try to fix this.

[maxd-nordic]

I just made a new release that should resolve the issue. Would you like to try it out? Make sure that your RTT application has an AT shell that can deal with newlines inside AT commands. There was a recent update to modem_shell to make that possible. The AT Shell library already has that feature for a while.