Blackfin: remove CONFIG_MEM_GENERIC_BOARD

MEM_GENERIC_BOARD depends on GENERIC_BOARD, but this flag was removed
in 4f25eb85, therefore all references
to it from the source can be removed.

Signed-off-by: Christian Dietrich <qy03fugy@stud.informatik.uni-erlangen.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>