board_r: use IS_ENABLED(CONFIG_NEEDS_MANUAL_RELOC) in board_init_r()

Drop CONFIG_NEEDS_MANUAL_RELOC ifdefs in board_init_r() and use
IS_ENABLED() instead. Also, use the MANUAL_RELOC() macro to update the
initcall pointers.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>