clapack: disable on blackfin

The "_" symbol prefix added by the blackfin toolchain causes "undefined
symbol" build failures when the compiler is trying to link a binary
with libblas and/or liblapack.

This patch also disables clapack reverse dependencies: armadillo.

Fixes:
  http://autobuild.buildroot.net/results/a28df0ba10803e6e54c2f8160bbd8190cba4d690/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>