defaultenv-2: add boot sequence ...

This allows to boot a sequence of boot entries until one succeeds.

boot sources can be passed in $global.boot.default, which is now treated
as a list. Also a list of boot entries can be specified as arguments
to the boot script. The entries can be:

- a plain filename from /env/boot/
- a full path to an arbitrary file
- a directory containing boot entries

With this this command:

boot net nand-ubi /env/boot.d

would first use the /env/boot/net entry, if this fails the /env/boot/nand-ubi
entry and if this also fails the files from /env/boot.d/ (which could also
be links to boot scripts)

To make the above the default, global.boot.default would be specified as:

global.boot.default="net nand-ubi /env/boot.d"

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

defenv2: comment setting default values in /env/config ...

There is no need to set again default values in /env/config
because they are already set in /env/bin/init,
that allows /env/config-board to change those default values
without being overwritten by /env/config.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

defenv-2: bring back /env/config ...

The idea of having /env/init/* scripts was to make the configuration
more flexible and customizable for boards. It turned out though that
people (including myself) do not find the place where they should
change these settings.

So this patch brings back /env/config for defenv-2. The individual
env/init/* scripts are removed and their content is added to
/env/init/config-board. This makes the values from /env/init/config-board
the board specific defaults which can be overwritten in /env/config.

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