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/MIP405_defconfig b/configs/MIP405_defconfig
index 254cd41..4614167 100644
--- a/configs/MIP405_defconfig
+++ b/configs/MIP405_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_DATE=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_MAC_PARTITION=y
 CONFIG_ISO_PARTITION=y
 # CONFIG_MMC is not set