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/atngw100_defconfig b/configs/atngw100_defconfig
index 1e85f0a..17db763 100644
--- a/configs/atngw100_defconfig
+++ b/configs/atngw100_defconfig
@@ -19,6 +19,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_MMC=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_SPI_FLASH=y