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/am3517_crane_defconfig b/configs/am3517_crane_defconfig
index 81fec1e..2a0987e 100644
--- a/configs/am3517_crane_defconfig
+++ b/configs/am3517_crane_defconfig
@@ -22,6 +22,7 @@
 # CONFIG_CMD_NFS is not set
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_JFFS2=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y