dropbear: add ed25519 for failsafe key

dropbear: add ed25519 for failsafe key

At least Fedora and RHEL 9 set RSAMinSize=2048, so when trying to use
failsafe, we get 'Bad server host key: Invalid key length'
To workaround the issue, we can use: ssh -o RSAMinSize=1024 ...

Generating 2048 bits RSA is extremely slow, so add ed25519.
We keep RSA 1024 to be as compatible as possible.

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>