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_defconfig b/configs/vct_premium_onenand_defconfig
index 2a4a055..ac9cf36 100644
--- a/configs/vct_premium_onenand_defconfig
+++ b/configs/vct_premium_onenand_defconfig
@@ -15,6 +15,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_SNTP=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y