use C0135 key "S2" for reset-to-defaults

[TG9541]

Holding C0135 key "S2" during start-up or reset should set the EEPROM values for Node-ID and baud rate to default values:

• Node-ID in EE_NODE ($4000) to 1 (holding register 0)
• Baud rate in EE_BAUD ($4002) to 0 (default rate 9600, holding register 1)

Hint: the holding register addresses for EEPROM values are currently 0..n

[TG9541]

@tealbrains: as discussed in issue #12