sg3-utils: disable for blackfin

Suffers from the usual symbol prefixing problems for bfin, it's probably
easy to fix, however no bfin has/will be used for storage apps since
they lack the usual ports for it. Fixes:
http://autobuild.buildroot.net/results/314/31447787e70cd351ce01b7a49ba029758d0c68e5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>