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/grasshopper_defconfig b/configs/grasshopper_defconfig
index 2944d73..024231d 100644
--- a/configs/grasshopper_defconfig
+++ b/configs/grasshopper_defconfig
@@ -14,4 +14,5 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
+CONFIG_CMD_JFFS2=y
 CONFIG_MTD_NOR_FLASH=y