Blackfin arch: Remove cruft - CONFIG_DEBUG_SERIAL_EARLY_INIT and DEBUG_KERNEL_START

Remove cruft - CONFIG_DEBUG_SERIAL_EARLY_INIT didn't work that well, and
DEBUG_KERNEL_START was just implmented poorly. Will replace with a new
checkin.

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