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/vct_premium_onenand_small_defconfig b/configs/vct_premium_onenand_small_defconfig
index 8eca0a5..6e649ec 100644
--- a/configs/vct_premium_onenand_small_defconfig
+++ b/configs/vct_premium_onenand_small_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_CMD_NET is not set
 # CONFIG_CMD_NFS is not set
 # CONFIG_CMD_MISC is not set
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 # CONFIG_ISO_PARTITION is not set
 CONFIG_SYS_NS16550=y