Users might want to generate the password while the target is offline.
We don't provide a dedicated host tool for that, but barebox sandbox can
be used. Document this possibility.

While at it, remove a misplaced single quite and slash in the help text.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>