allow user to run code in user_rf_pre_init.

add void to "C" functions.

example:
extern "C" void __run_user_rf_pre_init(void) {
    system_phy_set_rfoption(2);
}

see: #515 and #413

Author: Links2004

cores/esp8266/Esp.h

@@ -68,7 +68,7 @@ enum ADCMode {
     ADC_VDD = 255
 };
 
-#define ADC_MODE(mode) extern "C" int __get_adc_mode() { return (int) (mode); }
+#define ADC_MODE(mode) extern "C" int __get_adc_mode(void) { return (int) (mode); }
 
 typedef enum {
      FM_QIO = 0x00,

cores/esp8266/core_esp8266_phy.c

@@ -247,18 +247,26 @@ void user_rf_pre_init() {
     rtc_reg[30] = 0;
 
     system_set_os_print(0);
+    __run_user_rf_pre_init();
 }
 
-extern int __get_rf_mode()  __attribute__((weak));
-extern int __get_rf_mode()
+extern int __get_rf_mode(void)  __attribute__((weak));
+extern int __get_rf_mode(void)
 {
     return 0;  // default mode
 }
 
-extern int __get_adc_mode() __attribute__((weak));
-extern int __get_adc_mode()
+extern int __get_adc_mode(void) __attribute__((weak));
+extern int __get_adc_mode(void)
 {
     return 33; // default ADC mode
 }
 
+extern void __run_user_rf_pre_init(void) __attribute__((weak));
+extern void __run_user_rf_pre_init(void)
+{
+    return; // default do noting
+}
+
+