globalvar: make globalvar functions more consistent ...

Similar to the device parameter functions also make the globalvar
functions more consistent. This also adds support for readonly
globalvars and changes several existing globalvars which should
really be readonly to readonly.

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

bootchooser: export bootchooser_boot ...

Some boards that boot directly from C code do already know that
they want to boot from bootchooser and nothing else. For these
it's easiest to call bootchooser_boot directly, so export this
function.

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

bootchooser: register as bootentry provider ...

Instead of using a global function called by
bootentry_create_from_name(), register the bootchooser as bootentry
provider.

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

boot: add framework for redundant boot scenarios ...

There are several use cases where a redundant Linux system is needed. The
barebox bootchooser framework provides the building blocks to model different
use cases without the need to start from the scratch over and over again.

The bootchooser works on abstract boot targets, each with a set of properties
and implements an algorithm which selects the highest priority target to boot.

See the documentation contained in this patch for more information.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>