package/dropbear: add warning about volatile keys

When the rootfs is read-only, keys will be generated in a volatile
location, which is inherently bad as host keys will change on each boot,
rendering them virtually useless.

Add a warning so the user is at least aware of the issue.

Hide the rm output to avoid noisy output, now that we have a proper warning.

Move the starting message after the symlink-block, to avoid messages
collision. Move the umask as well, since /etc/dropbear/ may be world
readable; just the private host keys should be ?00 (and dropbear handles
that by itself).

[Peter: minor tweaks to commit message]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>