New internal clocks in boards.txt

[mistepien]

New internal clocks 0.5 MHz and 0.25 MHz - quite useful if no sleep mode is possible and high performance is not a relevant factor.

[mistepien]

New internal clocks - power save solution if no sleep mode is available and performance is not a key factor. In my case ATmega was doing the stuff every 4 ms and and clock 0.25 MHz was good enough.