Make dropbearkey as ssh-keygen

[stokito]

After merging the #248 the dropbearkey can act as an OpenSSH ssh-keygen replacement.
On the OpenWrt the dropbear is compiled into a single binary.
I tried locally to create a symlink the dropbearkey to ssh-keygen but the dropbear doesn't support the ssh-keygen:

# ln -s /usr/bin/dropbearkey /usr/bin/ssh-keygen 
# ssh-keygen 
Dropbear SSH multi-purpose v2022.82
Make a symlink pointing at this binary with one of the
following names or run 'dropbearmulti <command>'.
'dropbear' - the Dropbear server
'dropbearkey' - the key generator

Will you accept a PR that will support the ssh-keygen?

Once we do that I can send PRs to OpenWrt with the commits:

• stokito/openwrt@f62bc75
• openwrt/packages@9a3d317

[mkj]

Supporting ssh-keygen as a name makes sense for programs that expect that program name. So I'd be happy for a PR to dropbearkey.

I'm not sure if all distros would want to symlink it, it might cause confusion about other options like encryption etc not being supported. But that's up to packagers I guess.