Convert CONFIG_BOARD_TYPES to Kconfig

This converts the following to Kconfig:
   CONFIG_BOARD_TYPES

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mscc_serval_defconfig b/configs/mscc_serval_defconfig
index e423fc4..1c86c0d 100644
--- a/configs/mscc_serval_defconfig
+++ b/configs/mscc_serval_defconfig
@@ -16,6 +16,7 @@
 CONFIG_BOOTARGS="console=ttyS0,115200"
 CONFIG_LOGLEVEL=7
 CONFIG_DISPLAY_CPUINFO=y
+CONFIG_BOARD_TYPES=y
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_SYS_PROMPT="serval # "
 # CONFIG_CMD_BDI is not set