avr32: add ATAG_BOARDINFO

The ATAG_BOARDINFO is intended to hand over the information
bd->bi_board_number from u-boot to the kernel.

This piece of information can be used to implement some kind of board
identification while booting the kernel. Therefore it is placed in .initdata
section and can be accessed via the new symbol board_number only while
initializing the kernel.

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>
Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>