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/xpedite537x_defconfig b/configs/xpedite537x_defconfig
index c517b03..07fdfaf 100644
--- a/configs/xpedite537x_defconfig
+++ b/configs/xpedite537x_defconfig
@@ -18,6 +18,7 @@
 CONFIG_CMD_SNTP=y
 CONFIG_CMD_DATE=y
 # CONFIG_CMD_HASH is not set
+CONFIG_CMD_JFFS2=y
 CONFIG_SYS_FSL_DDR2=y
 CONFIG_DS4510=y
 # CONFIG_MMC is not set