Bootloader is to make firmware upgrade over the serial/USB possible.
Bootloader is always programmed and should not be programmed unless 
you screwed it up. It should be programmed w. an ISP-programmer and 
proper fuses for the AVR should be set. If you're hacking with this
you should be experienced.

This is stk500v2 based serial bootloaders for the RSS2 sensor boards.
There are two variants for the 128k and for 256k version of the board.


Check the the script Program-sensor.sh how write the bootloader also
set the ISP-programmar you have use in the script.

There are two variants:

* MCU on osci on  8MHz 
* MCU on XTAL on 16MHz 


Changes
2017-06-27 -- MCUSR saved in GPIOR0 so firmware can read 
Cause of boot.

2019-01-25 -- Bootloader at 115k UART speed