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

This is a re-commit of

  da05b143 ("x86/boot: Don't add the EFI stub to targets")

after the tagged patch incorrectly reverted it.

vmlinux-objs-y is added to targets, with an assumption that they are all
relative to $(obj); adding a $(objtree)/drivers/...  path causes the
build to incorrectly create a useless
arch/x86/boot/compressed/drivers/...  directory tree.

Fix this just by using a different make variable for the EFI stub.

Fixes: cb8bda8a

 ("x86/boot/compressed: Rename efi_thunk_64.S to efi-mixed.S")
Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: stable@vger.kernel.org # v6.1+
Link: https://lore.kernel.org/r/xm267ceukksz.fsf@bsegall.svl.corp.google.com