dropbear: add help text about key creation

Commit e7d04dd2 replaced /etc/dropbear with a symlink to /var/run and
updated the start scripts to replace it with a real directory, so the
keys would be persistent. However, it turns out that this is pretty
confusing even for expert users, who don't know how to make the keys
really persistent now.

Update the help text explaining what the issue is, and telling the user
to replace the /etc/dropbear symlink with a symlink to a persistent
directory. Also mention the possiblity of unionfs.

Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>