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/sheevaplug_defconfig b/configs/sheevaplug_defconfig
index 80cc62e..19acc7d 100644
--- a/configs/sheevaplug_defconfig
+++ b/configs/sheevaplug_defconfig
@@ -20,6 +20,7 @@
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_EXT4=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_CMD_UBI=y
 CONFIG_ISO_PARTITION=y
 CONFIG_SYS_NS16550=y