micropython: Disable for Blackfin

There are two problems building micropython for Blackfin. The first is
some printf format specifier warnings/errors that seem to be triggered
only for that architecture/compiler. This could be worked around by
specifying CFLAGS=-Wno-error=format.

The second problem is that libffi doesn't provide the closure
implementation on Blackfin. There is no known workaround for this issue.

For now disable micropython on Blackfin.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>