set default system tick to 15ms

Author: feilipu

src/FreeRTOSVariant.h

@@ -22,7 +22,7 @@ extern "C" {
 // System Tick - Scheduler timer
 // Use the Watchdog timer, and choose the rate at which scheduler interrupts will occur.
 
-#define portUSE_WDTO			WDTO_30MS			// portUSE_WDTO to use the Watchdog Timer for xTaskIncrementTick
+#define portUSE_WDTO			WDTO_15MS	// portUSE_WDTO to use the Watchdog Timer for xTaskIncrementTick
 
 /* Watchdog period options: 	WDTO_15MS
 								WDTO_30MS