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/sh7763rdp_defconfig b/configs/sh7763rdp_defconfig
index ec0275e..0286166 100644
--- a/configs/sh7763rdp_defconfig
+++ b/configs/sh7763rdp_defconfig
@@ -21,5 +21,6 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 # CONFIG_CMD_MISC is not set
+CONFIG_CMD_JFFS2=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_USE_PRIVATE_LIBGCC=y