avr: teensy 2.0 Non-Functional

The AVR Teensy 2.0 configuration (the only AVR target I have) does not seem functional. The two targets that supposedly should work are:

* hello -- `File "nuttx.hex". 30828 bytes, 96% used` This seems really high, and I do not get anything on the UART
* nsh -- `File "nuttx.hex". 53729 bytes, 167% used` Clearly non-functional.

We really need to decide if we keep these targets or not, because right now we are keeping them building but not making sure they are functional. I worry that the `hc` targets are in a similar place, there is some nostalgia for me to that port even though I have not used it with NuttX because the hcs12 was my first microcontroller, but I am not going to take ownership of that.

Start with the AVR Teensy 2.0 configuration and reproduce the `hello` and `nsh` builds described in the issue. Check the resulting `nuttx.hex` sizes and whether `hello` produces UART output; done means establishing functional targets or documenting that these targets should be removed, including the related `hc` concern if investigated.

Written by the indexing model from the issue text.