use sprintf-based implementation for dtostrf

altough the current implementation passed all the unit tests, an user reported problems with the actual code (http://forum.arduino.cc/index.php?topic=380422.0).
If libc has float support there is no need to do strange things like on AVR, so we can use the sprintf implementation

Author: facchinm

cores/arduino/stdlib_noniso.cpp

@@ -147,61 +147,9 @@ char* ultoa( unsigned long val, char *string, int radix )
     return string;
 }
 
-char * dtostrf(double number, signed char width, unsigned char prec, char *s) {
-
-    if (isnan(number)) {
-        strcpy(s, "nan");
-        return s;
-    }
-    if (isinf(number)) {
-        strcpy(s, "inf");
-        return s;
-    }
-
-    if (number > 4294967040.0 || number < -4294967040.0) {
-        strcpy(s, "ovf");
-        return s;
-    }
-
-    char* out = s;
-    int signInt_Part = 1;
-
-    // Handle negative numbers
-    if (number < 0.0) {
-        signInt_Part = -1;
-        number = -number;
-    }
-
-    // calc left over digits 
-    if (prec > 0)
-    {
-        width -= (prec + 1);
-    }
-
-    // Round correctly so that print(1.999, 2) prints as "2.00"
-    double rounding = 0.5;
-    for (uint8_t i = 0; i < prec; ++i)
-        rounding /= 10.0;
-
-    number += rounding;
-
-    // Extract the integer part of the number and print it
-    unsigned long int_part = (unsigned long)number;
-    double remainder = number - (double)int_part;
-    out += sprintf(out, "%*ld", width, int_part * signInt_Part);
-
-    // Print the decimal point, but only if there are digits beyond
-    if (prec > 0) {
-        *out = '.';
-        ++out;
-	// Copy character by character to 'out' string
-        for (unsigned char decShift = prec; decShift > 0; decShift--) {
-            remainder *= 10.0;
-            sprintf(out, "%d", (int)remainder);
-            out++;
-            remainder -= (double)(int)remainder;
-        }
-    }
-
-    return s;
-}
+char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
+    char fmt[20];
+    sprintf(fmt, "%%%d.%df", width, prec);
+    sprintf(sout, fmt, val);
+    return sout;
+}