[arch][microblaze] a bit of bugfixing and refactoring to get running on real hardware

Punched out a version of the microblaze on a ZYBO dev board, which required
a few tweaks an additions to run on real hardware.
Refactored the qemu and zybo microblaze target.

arch/microblaze/linker.ld

@@ -9,7 +9,7 @@ SECTIONS
     _start = .;
 
     /* vector table goes at 0, for qemu target, at least */
-    .vectors : AT(0) {
+    .vectors : AT(%VECTOR_BASE_PHYS%) {
         KEEP(*(.vectors))
     }