applications: serial_lte_modem: Allow skipping of startup message

If there is no certainty that remote end of serial connection has
configured baud rates and flow-control pins correctly, it might
be safer to just skip the "Ready" message from serial port.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

applications/serial_lte_modem/Kconfig

@@ -201,6 +201,12 @@ config SLM_MODEM_PIPE_TIMEOUT
 
 endif
 
+config SLM_SKIP_READY_MSG
+	bool "Skip ready message"
+	help
+	  Skip sending the ready message when the application is powered on.
+
+
 module = SLM
 module-str = serial modem
 source "${ZEPHYR_BASE}/subsys/logging/Kconfig.template.log_config"

applications/serial_lte_modem/overlay-cmux.conf

@@ -5,6 +5,7 @@
 #
 
 CONFIG_SLM_CMUX=y
+CONFIG_SLM_SKIP_READY_MSG=y
 
 # Zephyr modem subsystem
 CONFIG_MODEM_MODULES=y

applications/serial_lte_modem/src/slm_at_host.c

@@ -910,9 +910,12 @@ int slm_at_host_init(void)
 		return err;
 	}
 
-	err = slm_at_send_str(SLM_SYNC_STR);
-	if (err) {
-		return err;
+	if (!IS_ENABLED(CONFIG_SLM_SKIP_READY_MSG)) {
+		/* Send Ready string to indicate that AT host is ready */
+		err = slm_at_send_str(SLM_SYNC_STR);
+		if (err) {
+			return err;
+		}
 	}
 
 	/* This is here and not earlier because in case of firmware