Implement reset button support

[dmikushin]

• Solder reset button instead of a useless 78M05
• Connect reset button to GND and free ATTiny85 pin
• Implement signal callback from ATTiny85 to UART in case of long button press

[dmikushin]

• Long button press shall trigger button = true in ATTiny85 firmware
• b command shall return the button status as b0 or b1 and set button = false