Ports: Add preliminary dropbear port

This is very basic and doesn't support many features. Instead
of describing what it *doesn't* support, I'll describe what I
have tested:
1. Public key authentication (password is not supported)
2. Single command execution
3. PTY-less interactive bash shell (/bin/sh doesn't work)
4. Multi-user (i.e you can ssh as 'anon' as well as root)

Ports/dropbear/patches/fix-includes.patch

@@ -0,0 +1,11 @@
+Not sure why this is missing, but okay.
+--- a/includes.h.orig
++++ b/includes.h
+@@ -38,6 +38,7 @@
+ #include <sys/un.h>
+ #include <sys/wait.h>
+ #include <sys/resource.h>
++#include <sys/select.h>
+ 
+ #include <stdio.h>
+ #include <errno.h>