Simplify String::concat(char)

Now concat(const char*, unsigned int) no longer requires a
nul-terminated string, we can simplify the concat(char) method to just
pass the address of the single character instead of having to create
buffer with nul-termination.

Author: matthijskooijman

hardware/arduino/avr/cores/arduino/WString.cpp

@@ -279,10 +279,7 @@ unsigned char String::concat(const char *cstr)
 
 unsigned char String::concat(char c)
 {
-	char buf[2];
-	buf[0] = c;
-	buf[1] = 0;
-	return concat(buf, 1);
+	return concat(&c, 1);
 }
 
 unsigned char String::concat(unsigned char num)