Implement uart_pipe functionality for the modem so that it
integrates with the PPP implementation.
Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>
Create a driver for GSM modems that use a standard AT command set
and enable Zephyr's own PPP stack for IP traffic.
Signed-off-by: Patrik Flykt <patrik.flykt@intel.com>