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/fo300_defconfig b/configs/fo300_defconfig
index d51e31d..24470f3 100644
--- a/configs/fo300_defconfig
+++ b/configs/fo300_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_BSP=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_DIAG=y
 CONFIG_MAC_PARTITION=y
 CONFIG_ISO_PARTITION=y