gdb: disable on Blackfin

gdbserver does not build on Blackfin FLAT (undefined reference to
`_td_thr_tlsbase') and while it builds on Blackfin FDPIC, it's pretty
unlikely that it is going to work properly with the cross-gdb provided
by the Analog Devices toolchain: the Analog Devices toolchain provides
a 6.6 cross-gdb, while we use gdb 7.8.2 for gdbserver.

Therefore, let's simply disable gdb completely for Blackfin.

Fixes:

  http://autobuild.buildroot.org/results/2e5/2e5310d0e869f036ec70b978c62c7bd9d3d2bd70/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>