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>
diff --git a/configs/smdk5250_defconfig b/configs/smdk5250_defconfig
index a35e4fc..465a75a 100644
--- a/configs/smdk5250_defconfig
+++ b/configs/smdk5250_defconfig
@@ -1,3 +1,4 @@
 CONFIG_SPL=y
 +S:CONFIG_ARM=y
++S:CONFIG_ARCH_EXYNOS=y
 +S:CONFIG_TARGET_SMDK5250=y