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/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index 0bfcdd2..4a13700 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -28,6 +28,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_EMBED=y