exynos: kconfig: move board select menu and common settings

Becuase the board select menu in arch/arm/Kconfig is too big,
move the Exynos board select menu to exynos/Kconfig.

Consolidate also common settings (CONFIG_SYS_CPU="armv7" and
CONFIG_SYS_SOC="exynos").

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Minkyu Kang <mk7.kang@samsung.com>