key files are in a different location that a standard linux distro: look in /etc/dropbear for ssh files e.g.
root@antminer-6:~# ls -alt /etc/dropbear/
-rw-r--r-- 1 root root 1214 Apr 10 09:02 authorized_keys
drwxr-xr-x 1 root root 0 Feb 21 14:17 .
-rw------- 1 root root 459 Dec 1 20:28 dropbear_dss_host_key
-rw------- 1 root root 427 Dec 1 20:28 dropbear_rsa_host_key
drwxrwxr-x 1 root root 0 Dec 1 20:28 ..
root@antminer-6:~#
this is a handy bit of info..