microblaze: Add linux.bin.ub target

Currently the linux.bin target creates both linux.bin and linux.bin.ub.
Add linux.bin.ub as separate target to generate linux.bin.ub.

Signed-off-by: Jason Wu <huanyu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>