loadenv: allow more fine grained environment loading
This implements two new options for the loadenv command:
-s: removes (scrubs) old directory contents to be able to
create a fresh environment from for example /dev/defaultenv
-n: no overwrite. Do not overwrite existing files. This allows
to keep parts of the old environment.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>