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/lager_defconfig b/configs/lager_defconfig
index 1071da2..901b34e 100644
--- a/configs/lager_defconfig
+++ b/configs/lager_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SYS_MALLOC_F_LEN=0x8000
+CONFIG_ARCH_RMOBILE_BOARD_STRING="Lager"
 CONFIG_R8A7790=y
 CONFIG_TARGET_LAGER=y
 CONFIG_SPL_SERIAL_SUPPORT=y