Change DTR to RTS in lpc_ispmode

The pin used to determine if the microcontroller should enter bootloader mode is connected to RTS.

lpc.c

@@ -328,7 +328,7 @@ int lpc_read_partid(struct lpc_device *dev) {
 }
 
 void lpc_ispmode(struct lpc_device *dev, int state) {
-  serial_setline(dev->port, SERIAL_DTR, state);
+  serial_setline(dev->port, SERIAL_RTS, state);
 }
 
 void lpc_reset(struct lpc_device *dev, int isp) {