Allow having different board with the same chip.

	* config/avr/boards/shrimp/boardsupport.c (initialise_board): Call
	PIN_INIT.
	(start_trigger): Call PIN_SET.
	(stop_trigger): Call PIN_CLEAR.
	* config/avr/chips/atmega328p/chipsupport.h (PIN_INIT): New macro.
	(PIN_SET): Likewise.
	(PIN_CLEAR): Likewise.