app: scripts: Prefer to use socat instead of DD for serial capture

The DD utility is written only for block devices, not character
devices, so it might stop when reaching EOF.

Socat is a tool that is supposed to work on this job, but not
always installed.

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

Author: SeppoTakalo

app/scripts/sm_start_ppp.sh

@@ -239,8 +239,15 @@ test -c $AT_CMUX
 
 if [ $TRACE -gt 0 ]; then
 	echo "Starting trace collection..."
-	start-stop-daemon --start --pidfile $TRACE_PID_FILE --make-pidfile \
-	 --background --exec /bin/dd -- if=$MT_CMUX of=$MODEM_TRACE_FILE bs=1024
+	# Prefer to use socat, if installed.
+	if command -v socat >/dev/null 2>&1; then
+		start-stop-daemon --start --pidfile $TRACE_PID_FILE --make-pidfile \
+			--background --exec $(command -v socat) -- -u $MT_CMUX,cfmakeraw,clocal=1 \
+			CREATE:$MODEM_TRACE_FILE
+	else
+		start-stop-daemon --start --pidfile $TRACE_PID_FILE --make-pidfile \
+		--background --exec /bin/dd -- if=$MT_CMUX of=$MODEM_TRACE_FILE bs=1024
+	fi
 fi
 echo "Connect and wait for PPP link..."