env: Complete generic support for writable list

This completes what 890feeca started by selecting ENV_APPEND and
loading the default env before any other sources. This ensures that load
operations pick up all non-writable vars from the default env and only
permitted parts from other locations according to the regular
priorities.

With this change, boards only need to define the list of writable
variables but no longer have to provide a custom env_get_location
implementation.

CC: Joe Hershberger <joe.hershberger@ni.com>
CC: Marek Vasut <marex@denx.de>
CC: Stefan Herbrechtsmeier <stefan.herbrechtsmeier-oss@weidmueller.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Marek Vasut <marex@denx.de>