AP_HAL_SITL: flush data from select call as we don't wait select in AP_Periph

libraries/AP_HAL_SITL/CANSocketIface.cpp

@@ -469,6 +469,8 @@ bool CANIface::select(bool &read_select, bool &write_select,
 {
     // Detecting whether we need to block at all
     bool need_block = !write_select;    // Write queue is infinite
+    // call poll here to flush some tx
+    _poll(true, true);
 
     if (read_select && _hasReadyRx()) {
         need_block = false;

libraries/AP_HAL_SITL/Util.h

@@ -57,6 +57,7 @@ public:
 
 #if !defined(HAL_BUILD_AP_PERIPH)
     enum safety_state safety_switch_state(void) override;
+    void set_cmdline_parameters() override;
 #endif
 
     bool trap() const override {
@@ -70,7 +71,6 @@ public:
 #endif
     }
 
-    void set_cmdline_parameters() override;
 private:
     SITL_State *sitlState;