omap3: kconfig: move board select menu and common settings

Becuase the board select menu in arch/arm/Kconfig is too big,
move the OMAP3 board select menu to omap3/Kconfig.

Move also common settings (CONFIG_SYS_CPU="armv7" and
CONFIG_SYS_SOC="omap3

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/configs/dig297_defconfig b/configs/dig297_defconfig
index 9309ac2..95bc353 100644
--- a/configs/dig297_defconfig
+++ b/configs/dig297_defconfig
@@ -1,2 +1,3 @@
 CONFIG_ARM=y
+CONFIG_OMAP34XX=y
 CONFIG_TARGET_DIG297=y