toolchain-external: add hashes for Blackfin toolchains

Those toolchains are downloaded from Sourceforge, and are therefore
affected by the Sourcefoge download issues. Therefore, this commit
adds the hashes for those toolchain tarballs.

Fixes:

  http://autobuild.buildroot.org/results/fa5/fa5e38246dddd661f1d674f3521d21297796bce3/


  (and other similar issues)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>