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/devkit3250_defconfig b/configs/devkit3250_defconfig
index 8a8edf2..06874c9 100644
--- a/configs/devkit3250_defconfig
+++ b/configs/devkit3250_defconfig
@@ -22,6 +22,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_DM=y
 CONFIG_SPL_DM=y
 CONFIG_DM_GPIO=y