toolchain/gdb: disable support of full gdb on target for blackfin

There is no support in gdb 6.6 for the Blackfin architecture, so
disallow this choice. This fixes the following build failure:

  http://autobuild.buildroot.org/results/e37db29810265a756833da163fda444d967d7874/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>