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/porter_defconfig b/configs/porter_defconfig
index 64b064d..b04627e 100644
--- a/configs/porter_defconfig
+++ b/configs/porter_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="Porter"
 CONFIG_R8A7791=y
 CONFIG_TARGET_PORTER=y
 CONFIG_SPL_SERIAL_SUPPORT=y