spl: call spl_board_init() at the end of the spl init sequence

The spl_board_init() function on sandbox invokes the unit
tests. Invoking the tests should be done once the rest of the system
has been initialised. Call the spl_board_init() function at the very
end, once the rest of the initilisation functions have been called,
including the setting up of the LMB memory map.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>