rmobile: kconfig: move board select menu and common settings

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

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

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
diff --git a/configs/alt_defconfig b/configs/alt_defconfig
index 11a1c89..0655e60 100644
--- a/configs/alt_defconfig
+++ b/configs/alt_defconfig
@@ -1,2 +1,3 @@
 CONFIG_ARM=y
++S:CONFIG_RMOBILE=y
 CONFIG_TARGET_ALT=y