[ion] Encode the serial number in Base64

Also, expose serialNumber() in Ion and keep "copySerialNumber" in
Ion::Device. This allows the DFU bootloader to remain .bss-free.

ion/src/simulator/init.cpp

@@ -116,7 +116,3 @@ void Ion::msleep(long ms) {
     }
   }
 }
-
-void Ion::getSerialNumber(char * buffer) {
-  strlcpy(buffer, "Simulator", Ion::SerialNumberLength+1);
-}