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/mecp5123_defconfig b/configs/mecp5123_defconfig
index db52077..3937fe8 100644
--- a/configs/mecp5123_defconfig
+++ b/configs/mecp5123_defconfig
@@ -11,6 +11,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 # CONFIG_PCI is not set