arm64: fix boot image dependencies to not generate invalid images

I fixed boot image dependencies for arch/arm in commit 3939f334
("ARM: 8418/1: add boot image dependencies to not generate invalid
images").

I see a similar problem for arch/arm64; "make -jN Image Image.gz"
would sometimes end up generating bad images where N > 1.

Fix the dependency in arch/arm64/Makefile to avoid the race
between "make Image" and "make Image.*".
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/Makefile

@@ -95,7 +95,7 @@ boot := arch/arm64/boot
 Image: vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
-Image.%: vmlinux
+Image.%: Image
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
 zinstall install: