[rfc] add MK_TELNET_SSL as a build option

[Adrian Chadd]

The telnet library requires those routines to be defined by the
application, but when doing crunchgen style binaries that doesn't work
- two sets of each of those symbols are defined and things don't link.

The solution here is to create a struct with function pointers that
the application registers at startup time, rather than having the
library link /into/ the application.



-adrian