Convert CONFIG_CMD_JFFS2 to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_JFFS2

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mcx_defconfig b/configs/mcx_defconfig
index 349df80..09dbc78 100644
--- a/configs/mcx_defconfig
+++ b/configs/mcx_defconfig
@@ -29,6 +29,7 @@
 CONFIG_CMD_DATE=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SYS_NS16550=y