config: exynos5420: move non common configs to specific board files

The media for boot and environment is a board-specific feature, not a
processor-specific. This is same to console port number and  some
other addresses. This patch moves the that kinds of configs to each
board-specific files from the common config file for Exynos5420.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>