Convert CONFIG_CMD_IOLOOP to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_IOLOOP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/gdsys/mpc8308/Kconfig b/board/gdsys/mpc8308/Kconfig
index 9a1a3a2..cb29c25 100644
--- a/board/gdsys/mpc8308/Kconfig
+++ b/board/gdsys/mpc8308/Kconfig
@@ -23,3 +23,8 @@
 	default "strider"
 
 endif
+
+config CMD_IOLOOP
+	bool "Enable 'ioloop' and 'ioreflect' commands"
+	help
+	  These commands provide FPGA tests.