Add definitions for yield() and optimistic_yield()

Fixes: https://github.com/espressif/arduino-esp32/issues/54

cores/esp32/esp32-hal.h

@@ -33,6 +33,11 @@ extern "C" {
 #include <string.h>
 #include <math.h>
 
+//forward declaration from freertos/portmacro.h
+void vPortYield( void );
+#define yield() vPortYield()
+#define optimistic_yield(u)
+
 #define ESP_REG(addr) *((volatile uint32_t *)(addr))
 #define NOP() asm volatile ("nop")