AVR32: Initialize bi_flash* in board_init_r

The ATSTK1000-specific flash driver intializes bi_flashstart,
bi_flashsize and bi_flashoffset, but other flash drivers, like the CFI
driver, don't.

Initialize these in board_init_r instead so that things will still be
set up correctly when we switch to the CFI driver.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>