add board_vbus_set() for samd21/d51 to enable usb host vbus

enable host example build for samd21/d51

hw/bsp/samd5x_e5x/boards/feather_m4_express/board.h

@@ -65,6 +65,10 @@
 #define MAX3421_INTR_PIN        19 // D9
 #define MAX3421_INTR_EIC_ID     3  // EIC3
 
+static inline void board_vbus_set(uint8_t rhport, bool state) {
+ (void) rhport; (void) state;
+}
+
 #ifdef __cplusplus
  }
 #endif