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/xpedite517x_defconfig b/configs/xpedite517x_defconfig
index 6457d19..848d27e 100644
--- a/configs/xpedite517x_defconfig
+++ b/configs/xpedite517x_defconfig
@@ -16,6 +16,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_SNTP=y
 CONFIG_CMD_DATE=y
+CONFIG_CMD_JFFS2=y
 CONFIG_DS4510=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y