root@Omega-DE0B:/usr/bin# dropbearkey -h
Usage: dropbearkey -t <type> -f <filename> [-s bits]
-t type Type of key to generate. One of:
                rsa
-f filename    Use filename for the secret key.
               ~/.ssh/id_dropbear is recommended for client keys.
-s bits Key size in bits, should be a multiple of 8 (optional)
-y              Just print the publickey and fingerprint for the
                private key in <filename>.
root@Omega-DE0B:/usr/bin#

On Omega2 with an OpenWrt 18-06 based FW (eg. v0.3.2 b239) enable the fifth (5th) line of the '/etc/opkg/distfeeds.conf' file and run
opkg update
opkg install openssh-keygen

# Omega2+ FW v0.3.2 b239

root@Omega-5BE1:/# opkg update
Downloading http://downloads.openwrt.org/releases/18.06-SNAPSHOT/packages/mipsel_24kc/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/openwrt_packages
Downloading http://downloads.openwrt.org/releases/18.06-SNAPSHOT/packages/mipsel_24kc/packages/Packages.sig
Signature check passed.
 ...

root@Omega-5BE1:/# opkg install openssh-keygen
Installing openssh-keygen (7.7p1-2) to root...
Downloading http://downloads.openwrt.org/releases/18.06-SNAPSHOT/packages/mipsel_24kc/packages/openssh-keygen_7.7p1-2_mipsel_24kc.ipk
Configuring openssh-keygen.

root@Omega-5BE1:/# ssh-keygen --help
ssh-keygen: unrecognized option: -
usage: ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa]
                  [-N new_passphrase] [-C comment] [-f output_keyfile]
       ssh-keygen -p [-P old_passphrase] [-N new_passphrase] [-f keyfile]
       ssh-keygen -i [-m key_format] [-f input_keyfile]
       ssh-keygen -e [-m key_format] [-f input_keyfile]
       ssh-keygen -y [-f input_keyfile]
       ssh-keygen -c [-P passphrase] [-C comment] [-f keyfile]
       ssh-keygen -l [-v] [-E fingerprint_hash] [-f input_keyfile]
       ssh-keygen -B [-f input_keyfile]
       ssh-keygen -D pkcs11
       ssh-keygen -F hostname [-f known_hosts_file] [-l]
       ssh-keygen -H [-f known_hosts_file]
       ssh-keygen -R hostname [-f known_hosts_file]
       ssh-keygen -r hostname [-f input_keyfile] [-g]
       ssh-keygen -G output_file [-v] [-b bits] [-M memory] [-S start_point]
       ssh-keygen -T output_file -f input_file [-v] [-a rounds] [-J num_lines]
                  [-j start_line] [-K checkpt] [-W generator]
       ssh-keygen -s ca_key -I certificate_identity [-h] [-U]
                  [-D pkcs11_provider] [-n principals] [-O option]
                  [-V validity_interval] [-z serial_number] file ...
       ssh-keygen -L [-f input_keyfile]
       ssh-keygen -A
       ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]
                  file ...
       ssh-keygen -Q -f krl_file file ...

Good luck!