AVR32: Split start_u_boot into board_init_f and board_init_r

Split the avr32 initialization code into a function to run before
relocation, board_init_f and a function to run after relocation,
board_init_r. For now, board_init_f simply calls board_init_r
at the end.

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