arm:board:exynos4: add CONFIG_SYS_GENERIC_BOARD

Add CONFIG_SYS_GENERIC_BOARD for all Exynos4 boards.

Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Cc: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>