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>

defaultenv: Allow multiple defaultenvironment overlays ...

We can compile barebox for multiple boards at once, but currently
they all share a single default environment. This patch adds a
defaultenv_append() which boards can call to customize the default
environment during runtime. Each board now generate default environment
snippets using bbenv-y and add them during runtime with defaultenv_append()

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