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/remove-setrlimit.patch

@@ -0,0 +1,12 @@
+--- a/dbutil.c.orig
++++ b/dbutil.c
+@@ -553,9 +553,6 @@
+ }
+ 
+ void disallow_core() {
+-	struct rlimit lim;
+-	lim.rlim_cur = lim.rlim_max = 0;
+-	setrlimit(RLIMIT_CORE, &lim);
+ }
+ 
+ /* Returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE, with the result in *val */