Commit for Allow specifying authorized_keys directory.

[HansH111]

Why not specify a different authorized_keys file, it should be much simpler to implement.
No need to do all the stuff for a possible ~ expansion.

Just dropbear -H ~/.ssh -D auth_test
That way you can also have multiple dropbear active with each own authorized_keys file

[fjh1997]

see：#330

[HansH111]

Not only the directory but also specify the authorized_keys filename.
That way you can have multiple dropbear listening on different ports all within the same directory but with a different authorized_keys file.