arm64: Add BOOT_TARGETS variable

Add a new variable containing a list of possible targets. Mark them as
phony. This matches the approach taken for arch/arm

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Link: https://lore.kernel.org/r/20240329032836.141899-2-sjg@chromium.org
Signed-off-by: Will Deacon <will@kernel.org>

Author: sjg20

arch/arm64/Makefile

@@ -154,6 +154,10 @@ libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
 # Default target when executing plain make
 boot		:= arch/arm64/boot
 
+BOOT_TARGETS	:= Image vmlinuz.efi
+
+PHONY += $(BOOT_TARGETS)
+
 ifeq ($(CONFIG_EFI_ZBOOT),)
 KBUILD_IMAGE	:= $(boot)/Image.gz
 else
@@ -163,7 +167,7 @@ endif
 all:	$(notdir $(KBUILD_IMAGE))
 
 vmlinuz.efi: Image
-Image vmlinuz.efi: vmlinux
+$(BOOT_TARGETS): vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
 Image.%: Image