Add MacAddress string constructor

Author: lucasssvaz

cores/esp32/MacAddress.h

@@ -54,6 +54,7 @@ class MacAddress : public Printable {
     MacAddress(const uint8_t *macbytearray) : MacAddress(MAC6, macbytearray) {}
 
     MacAddress(const char *macstr);
+    MacAddress(const String &macstr) : MacAddress(macstr.c_str()) {}
 
     virtual ~MacAddress() {}