remove some warnings

cores/esp8266/core_esp8266_noniso.c

@@ -34,12 +34,12 @@ int atoi(const char* s) {
 }
 
 long atol(const char* s) {
-    const char * tmp;
+    char * tmp;
     return strtol(s, &tmp, 10);
 }
 
 double atof(const char* s) {
-    const char * tmp;
+    char * tmp;
     return strtod(s, &tmp);
 }
 
@@ -148,7 +148,6 @@ char* ultoa(unsigned long value, char* result, int base) {
 }
 
 char * dtostrf(double number, signed char width, unsigned char prec, char *s) {
-    size_t n = 0;
 
     if(isnan(number)) {
         strcpy(s, "nan");

cores/esp8266/i2c.cpp

@@ -202,7 +202,8 @@ uint8_t twi_writeTo(uint8_t addr, uint8_t* data, uint8_t size, uint8_t wait, uin
 }
 
 uint8_t twi_transmit(const uint8_t* data, uint8_t length) {
-
+    //TODO implement twi_transmit
+    return 0;
 }
 
 void twi_attachSlaveRxEvent(void (*)(uint8_t*, int)) {

libraries/ESP8266WiFi/src/WiFiClient.cpp

@@ -157,7 +157,6 @@ int ICACHE_FLASH_ATTR WiFiClient::available()
 
 int ICACHE_FLASH_ATTR WiFiClient::read() 
 {
-    uint8_t b;
     if (!available())
         return -1;
 

libraries/ESP8266WiFi/src/WiFiUdp.cpp

@@ -54,6 +54,7 @@ WiFiUDP& WiFiUDP::operator=(const WiFiUDP& rhs)
     _ctx = rhs._ctx;
     if (_ctx)
         _ctx->ref();
+    return *this;
 }
 
 WiFiUDP::~WiFiUDP()

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -49,6 +49,7 @@ class ClientContext {
 
         ClientContext* next(ClientContext* new_next) {
             _next = new_next;
+            return _next;
         }
 
         void ref() {