Blackfin arch: Clean up trace buffer handling, No major functional changes.

Turns on trace earlier, so crashes at kernel start should print out a
trace, making things easier to debug.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>