gcc 8.4.0 throws format-errors for the sprintf statements

[gtjoseph]

The toolchain shipped with ESP-IDF has g++ version 8.4.0 which emits overflow errors when it thinks there's a possibility of overrunning the static buffer given to sprintf. In practical terms, there's no possibility of that but the compiler doesn't know that. Adding the "hh" hint to all the "%02d" format strings will address the error.