Provide selection between A0 and VCC (#443, #338)

Author: igrr

README.md

@@ -169,6 +169,16 @@ Several APIs may be used to get flash chip info:
 
 ```ESP.getCycleCount()``` returns the cpu instruction cycle count since start as an unsigned 32-bit.  This is useful for accurate timing of very short actions like bit banging.
 
+```ESP.getVcc()``` may be used to measure supply voltage. ESP needs to reconfigure the ADC
+at startup in order for this feature to be available. Add the following line to the top
+of your sketch to use ```getVcc```:
+```
+ADC_MODE(ADC_VCC);
+```
+TOUT pin has to be disconnected in this mode.
+
+Note that by default ADC is configured to read from TOUT pin using ```analogRead(A0)```, and 
+```ESP.getVCC()``` is not available.
 
 #### OneWire (from https://www.pjrc.com/teensy/td_libs_OneWire.html) ####
 

hardware/esp8266com/esp8266/cores/esp8266/Esp.h

@@ -61,6 +61,15 @@ enum RFMode {
 #define WAKE_NO_RFCAL    RF_NO_CAL
 #define WAKE_RF_DISABLED RF_DISABLED
 
+enum ADCMode {
+    ADC_TOUT = 33,
+    ADC_TOUT_3V3 = 33,
+    ADC_VCC = 255,
+    ADC_VDD = 255
+};
+
+#define ADC_MODE(mode) extern "C" int __get_adc_mode() { return (int) (mode); }
+
 typedef enum {
      FM_QIO = 0x00,
      FM_QOUT = 0x01,

hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_phy.c

@@ -224,6 +224,7 @@ static uint8_t phy_init_data[128] =
 
 extern int __real_register_chipv6_phy(uint8_t* init_data);
 extern int __wrap_register_chipv6_phy(uint8_t* unused) {
+    phy_init_data[107] = __get_adc_mode();
     return __real_register_chipv6_phy(phy_init_data);
 }
 
@@ -243,6 +244,10 @@ extern int __get_rf_mode()
     return 0;  // default mode
 }
 
-
+extern int __get_adc_mode() __attribute__((weak));
+extern int __get_adc_mode()
+{
+    return 33; // default ADC mode
+}