target-microblaze: Tidy up the base-vectors property

Rename the "xlnx.base-vectors" string to "base-vectors" and
move the base_vectors variable into the cfg struct.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>