ARM: rmobile: Convert CONFIG_ARCH_RMOBILE_BOARD_STRING to Kconfig

Convert the symbol to Kconfig, no functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
diff --git a/configs/kzm9g_defconfig b/configs/kzm9g_defconfig
index f34fe9b..af93185 100644
--- a/configs/kzm9g_defconfig
+++ b/configs/kzm9g_defconfig
@@ -2,6 +2,7 @@
 # CONFIG_SYS_THUMB_BUILD is not set
 CONFIG_ARCH_RMOBILE=y
 CONFIG_SYS_TEXT_BASE=0x00000000
+CONFIG_ARCH_RMOBILE_BOARD_STRING="KMC KZM-A9-GT"
 CONFIG_TARGET_KZM9G=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y