setools: Disable on Blackfin and NIOS II

The setools package is not currently building on either the NIOS II or
the Blackfin architectures. Disabling for now.

Fixes:
http://autobuild.buildroot.net/results/2ee84c0dc027912e059ca4ae518d6f11fd8317a7/
http://autobuild.buildroot.net/results/401e4c48f4d865b2722f284f1038e25c5d420f43/
http://autobuild.buildroot.net/results/e67ef47ea9ff4cbb012d374b3b290fb7bddf7ef3/



[Thomas: add comment explaining the dependencies.]

Signed-off-by: Clayton Shotwell <clayton.shotwell@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>