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/km_kirkwood_128m16_defconfig b/configs/km_kirkwood_128m16_defconfig
index 0884688..df7dac8 100644
--- a/configs/km_kirkwood_128m16_defconfig
+++ b/configs/km_kirkwood_128m16_defconfig
@@ -19,6 +19,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y