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

Consolidate also common settings (CONFIG_SYS_CPU="arm926ejs" and
CONFIG_SYS_SOC="kirkwood").
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: NPrafulla Wadasdkar <prafulla@marvell.com>
Cc: Luka Perkov <luka@openwrt.org>