Add atof implementation

Fix #27

cores/esp8266/WString.cpp

@@ -702,12 +702,12 @@ void ICACHE_FLASH_ATTR String::trim(void)
 
 long ICACHE_FLASH_ATTR String::toInt(void) const
 {
-	if (buffer) return atol_internal(buffer);
+	if (buffer) return atol(buffer);
 	return 0;
 }
 
 float ICACHE_FLASH_ATTR String::toFloat(void) const
 {
-	if (buffer) return float(atof_internal(buffer));
+	if (buffer) return atof(buffer);
 	return 0;
 }

cores/esp8266/core_esp8266_noniso.c

@@ -26,7 +26,7 @@
 #define sprintf ets_sprintf
 #define strcpy ets_strcpy
 
-long atol_internal(const char* s)
+long atol(const char* s)
 {
   int result = 0;
   int i;
@@ -44,10 +44,52 @@ long atol_internal(const char* s)
   return sign * result;
 }
 
-float atof_internal(const char* s)
+// Source:
+// https://github.com/anakod/Sming/blob/master/Sming/system/stringconversion.cpp#L93
+double atof(const char* s)
 {
-  float result = 0;
-  return result;
+  double result = 0;
+  double sign = 1;
+
+  while (*s == ' ' || *s == '\t' || *s == '\r' || *s == '\n')
+    ++s;
+
+  if (*s == 0)
+    return 0;
+
+  if (*s == '-') 
+  {
+    sign = -1;
+    ++s;
+  }
+  if (*s == '+')
+  {
+    ++s;
+  }
+
+  bool decimals = false;
+  double factor = 1.0;
+  char c;
+  while((c = *s)) 
+  {
+    if (c == '.') 
+    {
+      decimals = true;
+      ++s;
+      continue;
+    }
+
+    int d = c - '0';
+    if (d < 0 || d > 9)
+      break;
+
+    result = 10.0 * result + d;
+    if (decimals)
+      factor *= 0.1;
+    ++s;
+  }
+
+  return result * factor;
 }