x86, boot: Use the usual -y -n mechanism for objects in vmlinux

Switch VMLINUX_OBJS to vmlinux-objs-y, to eliminate Makefile
conditionals in favor of vmlinux-objs-$(CONFIG_*) constructs.

This does not change the generated code at all.
Signed-off-by: Josh Triplett <josh@joshtriplett.org>

arch/x86/boot/compressed/Makefile

@@ -26,18 +26,16 @@ LDFLAGS_vmlinux := -T
 hostprogs-y	:= mkpiggy
 HOST_EXTRACFLAGS += -I$(srctree)/tools/include
 
-VMLINUX_OBJS = $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
+vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
 	$(obj)/string.o $(obj)/cmdline.o $(obj)/early_serial_console.o \
 	$(obj)/piggy.o $(obj)/cpuflags.o $(obj)/aslr.o
 
 $(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar -mno-red-zone
 
-ifeq ($(CONFIG_EFI_STUB), y)
-	VMLINUX_OBJS += $(obj)/eboot.o $(obj)/efi_stub_$(BITS).o \
-				$(objtree)/drivers/firmware/efi/libstub/lib.a
-endif
+vmlinux-objs-$(CONFIG_EFI_STUB) += $(obj)/eboot.o $(obj)/efi_stub_$(BITS).o \
+	$(objtree)/drivers/firmware/efi/libstub/lib.a
 
-$(obj)/vmlinux: $(VMLINUX_OBJS) FORCE
+$(obj)/vmlinux: $(vmlinux-objs-y) FORCE
 	$(call if_changed,ld)
 	@:
 
@@ -45,7 +43,7 @@ OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
 $(obj)/vmlinux.bin: vmlinux FORCE
 	$(call if_changed,objcopy)
 
-targets += $(patsubst $(obj)/%,%,$(VMLINUX_OBJS)) vmlinux.bin.all vmlinux.relocs
+targets += $(patsubst $(obj)/%,%,$(vmlinux-objs-y)) vmlinux.bin.all vmlinux.relocs
 
 CMD_RELOCS = arch/x86/tools/relocs
 quiet_cmd_relocs = RELOCS  $@