defaultenv: Reorder overlay append order ...

The defaultenv_add_base() functions adds the enviroment coming from
CONFIG_DEFAULT_ENVIRONMENT_PATH. But this is not only for
traditional enviroments, but also for external enviroments in e.g. build
systems. The external enviroment should override the enviroment set
in the board file and not the other way around. Made sure that
external enviroment is always added last.

Signed-off-by: Teresa Gámez <t.gamez@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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>