Blackfin arch: Remove cruft - CONFIG_DEBUG_SERIAL_EARLY_INIT didn't work that...

Blackfin arch: Remove cruft - CONFIG_DEBUG_SERIAL_EARLY_INIT didn't work that well with DEBUG_KERNEL_START

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