AP_HAL_Linux: RCInput_RPI: provide signal information

Otherwise it's difficult to know what signal is causing the program to
exit.

libraries/AP_HAL_Linux/RCInput_RPI.cpp

@@ -264,7 +264,7 @@ void RCInput_RPI::stop_dma()
 void RCInput_RPI::termination_handler(int signum)
 {
     stop_dma();
-    AP_HAL::panic("Interrupted");
+    AP_HAL::panic("Interrupted: %s", strsignal(signum));
 }