Replace Dropbear to OpenSSH + SFTP

The vanilla OpenWrt out of the box has a small Dropbear SSH server. But it doesn't support the SFTP and lacks of some other features. You can install another OpenSSH server which is bigger but has more features and default on desktop systems like Ubuntu. Many routers with OpenWrt as a stock firmware use it out of the box. So for this routers you really don't need anything to do and just start using it.

Set a Dropbear's port to some unused (e.g. 2222) and restart it

uci set dropbear.@dropbear[0].Port=2222
uci commit dropbear
/etc/init.d/dropbear restart

Ensure that the root user has a password by using the
```
passwd
```
command.

Reconnect to the SSH using the new port
```
ssh root@192.168.1.1 -p 2222
```

Install OpenSSH server
```
opkg update
opkg install openssh-server
```

Enable it and start

/etc/init.d/sshd enable
/etc/init.d/sshd start

The OpenSSH now use the standard 22 port. Reconnect to SSH over the 22 port
```
ssh root@192.168.1.1
```

Now you can disable the Dropbear

/etc/init.d/dropbear disable
/etc/init.d/dropbear stop

Install openssh-sftp-server package to support the SFTP protocol
```
opkg update
opkg install openssh-sftp-server
```

If needed, configure the OpenSSH server in /etc/ssh/sshd_config and restart it
```
/etc/init.d/sshd restart
```