defaultenv: Add boot option for DFU ...

DFU is for device firmware upgrade, but for development purposes it's
sometmes useful to just start a kernel vie DFU. This adds a boot option
for doing this and also the corresponding counterpart on the host. With
this it's possible to boot a system with:

scripts/dfuboot.sh -k linuximage -d dtb -c "root=ubi0:root ubi.mtd=ubi rootfstype=ubifs ignore_loglevel"

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>