leveldb: disable for blackfin

Fixes:
http://autobuild.buildroot.net/results/f79/f7930231cadeef7ccb0df6acf41b62c2c6fd1e02/



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