move yield to function

fixes: https://github.com/espressif/arduino-esp32/issues/193

cores/esp32/esp32-hal-misc.c

@@ -62,6 +62,11 @@ void arduino_phy_init()
 }
 #endif
 
+void yield()
+{
+    vPortYield();
+}
+
 uint32_t IRAM_ATTR micros()
 {
     uint32_t ccount;

cores/esp32/esp32-hal.h

@@ -39,8 +39,8 @@ extern "C" {
 #endif
 
 //forward declaration from freertos/portmacro.h
-void vPortYield( void );
-#define yield() vPortYield()
+void vPortYield(void);
+void yield(void);
 #define optimistic_yield(u)
 
 #define ESP_REG(addr) *((volatile uint32_t *)(addr))