microblaze: fix race condition in building boot images

I fixed a race condition in the parallel building of ARM in commit
3939f334 ("ARM: 8418/1: add boot image dependencies to not
generate invalid images").

I see the same problem for MicroBlaze too.

"make -j<N> ARCH=microblaze all linux.bin.ub" results in a broken build
because two threads descend into arch/microblaze/boot simultaneously.

Add proper dependencies to avoid it.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/Makefile

@@ -83,7 +83,9 @@ archheaders:
 	$(Q)$(MAKE) $(build)=arch/microblaze/kernel/syscalls all
 
 PHONY += linux.bin linux.bin.gz linux.bin.ub
-linux.bin linux.bin.gz linux.bin.ub: vmlinux
+linux.bin.ub linux.bin.gz: linux.bin
+linux.bin: vmlinux
+linux.bin linux.bin.gz linux.bin.ub:
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 	@echo 'Kernel: $(boot)/$@ is ready' ' (#'`cat .version`')'