dropbear: use ssh-keygen as an alias for dropbearkey

dropbear: use ssh-keygen  as an alias for dropbearkey

The DropBear's dropbearkey supports limited set of arguments of
OpenSSH ssh-keygen:  -t, -q -N -Y
After the change you can generate a key with the same command.
Still many features of the original OpenSSH ssh-keygen are absent in
the dropbearkey.
If it's needed then users should install openssh-keygen package that
will replace the /usr/bin/ssh-keygen with the full version.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/14174
[ wrap commit description to 80 columns ]
Link: https://github.com/openwrt/openwrt/pull/14174
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>