Blackfin arch: merge kgdb test code using common CONFIG_KGDB_TESTS

[Grace Pan <grace.pan@analog.com>: Add case for kgdb test in l1 and l2]

Signed-off-by: Grace Pan <grace.pan@analog.com>
Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>