sparc: leon2: Updates for generic board initialization

Reworked the LEON2 start.S code to call board_init_f function at startup.
Also implemented the relocate_code function in assembly to relocate the
monitor and setup the stack pointer before calling relocated board_init_r.

Add the CONFIG_SYS_GENERIC_BOARD variable to all the LEON2 boards.

Signed-off-by: Francois Retief <fgretief@spaceteq.co.za>