- ATmega based boards: Added parameter for W5100 to board_common.h
- Arduino Due based boards: Use board.h instead of providing w5100_params.h

- correct number of timers for atmega328p from 2 to 1
- correct number of timer channels for atmega328p from 3 to 2
- adapt atmega periph timer implementation accordingly

The `atmega_set_prescaler` was using a "sensible" default
but it's better to define it at the board level to make
it clear.

This adds a LED_PANIC macro which defines which LED,
or combination of LEDs should notify a panic error.
This is currently used to signal BADISR_vect errors.

- replace brief with name where required
- remove some unwanted empty lines
- fix missing closing doxygen name block