arch: remove Blackfin architecture

arch: remove Blackfin architecture

The Blackfin architecture has for a long time been complicated to
maintain, with poor support in upstream binutils/gcc. As of April
2018, the Blackfin architecture has been dropped from the upstream
Linux kernel. Also, the Analog Device engineer who used to be in touch
with the Buildroot community also privately said we should drop the
support for this architecture, which Analog Devices is no longer
using, promoting and maintaining.

The BR2_BINFMT_FLAT_SEP_DATA option becomes unselectable, it will be
removed in a future commit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>