Empty BSP architecture removal

[Ralph Holmes]

Hello,

I'm currently working on resolving #2442, which involves the removal of 
the avr/avrtest BSP family. However, since avrtest is the only BSP 
family for the avr architecture, I am unsure as to whether I should 
remove the avr directory entirely, or to just remove the avrtest BSP family.

Advice from someone with greater experience than I would be greatly 
appreciated.

Thanks,
- Ralph