mpc8315e-rdb: add the jffs2 image support

mpc8315e-rdb: add the jffs2 image support

There is a 32M NAND flash on this board, so it would be convenient
that a jffs2 image is created by default. Also change the default image
type from "tar.gz" to "tar.bz2" in order to be compatible with
the beaglebone black and edgerouter BSP.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>