x86/boot: Don't add the EFI stub to targets

vmlinux-objs-y is added to targets, which currently means that the EFI
stub gets added to the targets as well. It shouldn't be added since it
is built elsewhere.

This confuses Makefile.build which interprets the EFI stub as a target
	$(obj)/$(objtree)/drivers/firmware/efi/libstub/lib.a
and will create drivers/firmware/efi/libstub/ underneath
arch/x86/boot/compressed, to hold this supposed target, if building
out-of-tree. [0]

Fix this by pulling the stub out of vmlinux-objs-y into efi-obj-y.

[0] See scripts/Makefile.build near the end:
    # Create directories for object files if they do not exist
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lkml.kernel.org/r/20200715032631.1562882-1-nivedita@alum.mit.edu

arch/x86/boot/compressed/Makefile

@@ -90,8 +90,8 @@ endif
 
 vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
 
-vmlinux-objs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
 vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
+efi-obj-$(CONFIG_EFI_STUB) = $(objtree)/drivers/firmware/efi/libstub/lib.a
 
 # The compressed kernel is built with -fPIC/-fPIE so that a boot loader
 # can place it anywhere in memory and it will still run. However, since
@@ -115,7 +115,7 @@ endef
 quiet_cmd_check-and-link-vmlinux = LD      $@
       cmd_check-and-link-vmlinux = $(cmd_check_data_rel); $(cmd_ld)
 
-$(obj)/vmlinux: $(vmlinux-objs-y) FORCE
+$(obj)/vmlinux: $(vmlinux-objs-y) $(efi-obj-y) FORCE
 	$(call if_changed,check-and-link-vmlinux)
 
 OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S