microblaze: factored out common boot code

factored out the copy-pasted common boot code from the two microblaze platforms
into a dedicated microblaze bootloader (microblaze_boot.o).

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>