blackfin: adjust available ABIs

The FLAT (Separate Data) and FLAT Shared ABIs are rarely used, and the
FLAT Shared ABI requires the user to manually assign an unique ID to
each shared library, which we will never support in
Buildroot. Therefore, restrict ourselves to FLAT and FDPIC.

In addition to this, ensure that when FLAT is selected, only static
libraries are produced, because this is what FLAT supports. It will
fix problems such as
http://autobuild.buildroot.org/results/2d756d75162e8737e99df8189bde93ed1a09feef/build-end.log

.

Moreover, we make FDPIC the default ABI, since if someone is using
Buildroot, it's most likely to generate a fairly elaborate embedded
Linux system, on which shared libraries are probably useful.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>