This moves the variable assignments previously done in /env/config-board
to non volatile variables in /env/nv/. This makes the settings adjustable
by the user without editing a file.

Most of the changes are simple conversions which for many boards makes
/env/config-board unnecessary. Some boards had some logic to assign
global.boot.default based on the current bootsource. This has been
moved to /env/init/bootsource. An additional check is added to not
overwrite a nv.boot.default should it exist.

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