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/kmcoge4_defconfig b/configs/kmcoge4_defconfig
index 493d971..9a15d1e 100644
--- a/configs/kmcoge4_defconfig
+++ b/configs/kmcoge4_defconfig
@@ -24,6 +24,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 # CONFIG_CMD_HASH is not set
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 CONFIG_DOS_PARTITION=y
 # CONFIG_MMC is not set