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/atstk1002_defconfig b/configs/atstk1002_defconfig
index 49182e3..0fa7969 100644
--- a/configs/atstk1002_defconfig
+++ b/configs/atstk1002_defconfig
@@ -17,5 +17,6 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_MMC=y
 CONFIG_MTD_NOR_FLASH=y