python: pyA20 added different GPIO mappings

Unfortunately there is a difference between different boards when using pyA20
- the GPIO mappings.
Olimex tried to solve this by providing different packages for different boards.
Just made the receipe use different mappings.h, so it can be easily adjusted
to different boards in one receipe.

Signed-off-by: Jens Lucius <info@jenslucius.com>