CONFIG: EXYNOS5: Replace misnomer SMDK5250 with EXYNOS5250 and update Makefiles

Update the Makefiles so that all boards can use the same spl generation tool

Signed-off-by: Inderpal Singh <inderpal.singh@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>