Add xmega-a3bu Xplained target

	* configure: Regenerate
	* configure.ac: Add ATXMEGA256A3BUXPLD platform
	* src/platformcode/platformcode.c: Add trigger
Merge remote-tracking branch 'origin/atxmega256a3buxpld' into jp

Conflicts:
	ChangeLog
	configure
	configure.ac
Changed the name of the xmega platform.

From the chip name to the development board's name. Platform was
atxmega256a3buxpld, is now xmegaa3buxplained.

    * configure: Regenerated
    * configure.ac: Changed the name of the xmega platform from the chip
        name to the development board's name. Platform was
        atxmega256a3buxpld, is now xmegaa3buxplained.