openssh: allow configuration of hostkey type

openssh: allow configuration of hostkey type

Allow selection of host key types used by openssh via PACKAGECONFIG.
Any combination of hostkey-rsa, hostkey-ecdsa and hostkey-ed25519 can be
specified. Default to just generating ecdsa keys.

The current default generates all three keys. This can take a
significant amount of time on first boot. Having all three keys does not
significantly increase compatability. Also RSA keys are being deprecated
as they are no longer considered secure. Using just an ecdsa key reduces
key generation time by roughly 75%.

Signed-off-by: Matthew Bullock <mbullock@thegoodpenguin.co.uk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>