If you only need this for testing and debugging, why do you need it to be secure? You should not be debugging live systems while they are in a potentially hostile environment where a secure protocol would be needed. Considering the fact that even the very lightest secure protocol would eat up a significant portion of system memory, it would be silly to allocate such a high amount of resources for just debugging and testing. What you should do instead is only test your devices when you have physical control over them, ideally over a physical network link.
32 kB of RAM is really pushing it. Even Dropbear cannot run on a system that limited. Even doing TLS will be difficult, as WolfSSL may barely fit. The WolfSSL SSH library requires a minimum of 33 kB. It's likely that you'll need to develop this yourself, which should not be too difficult if you are using FreeRTOS. There is likely nothing that you can simply drop in to a 32 kB system.
If you are using Contiki as an RTOS, you may be able to use this TLS/DTLS implementation. It requires slightly more memory than 32 kB, but you should be able to strip it down sufficiently if you are familiar with embedded programming and optimization and don't need all the features.
When you begin to use systems this constrained, each individual system will have different priorities. As a result, no one creates a one-size-fits-all library for something as complicated as a secure remote access protocol. You are generally expected to be able to implement it yourself to fit your exact needs, either from scratch or by forking an extremely lightweight library. If you are not likely able to do this, you may have to rethink either your hardware and switch to something other than FreeRTOS, or your threat model (and communicate with the system over a secure medium).
