HAL_ChibiOS: default RTS pins to PULLDOWN

this avoids issues with SiK and RFD900x radios getting stuck in
bootloader mode due to a high RTS pin on power on.

We did this for Pixhawk6C in this PR:
ArduPilot#24169

this now applies it to all boards

libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py

@@ -411,10 +411,15 @@ def get_PUPDR_value(self):
                 self.type.startswith('UART')) and (
                 (self.label.endswith('_TX') or
                  self.label.endswith('_RX') or
-                 self.label.endswith('_CTS') or
-                 self.label.endswith('_RTS'))):
+                 self.label.endswith('_CTS'))):
                 v = "PULLUP"
 
+            # pulldown on RTS to prevent radios from staying in bootloader
+            if (self.type.startswith('USART') or
+                self.type.startswith('UART')) and (
+                 self.label.endswith('_RTS')):
+                v = "PULLDOWN"
+
             if (self.type.startswith('SWD') and
                     'SWDIO' in self.label):
                 v = "PULLUP"