efi/libstub: Avoid linking libstub/lib-ksyms.o into vmlinux

drivers/firmware/efi/libstub/Makefile builds a static library, which
is not linked into the main vmlinux target in the ordinary way [arm64],
or at all [ARM, x86].

Since commit:

  7f2084f ("[kbuild] handle exports in lib-y objects reliably")

any Makefile using lib-y generates lib-ksyms.o which is linked into vmlinux.

In this case, the following garbage object is linked into vmlinux.

  drivers/firmware/efi/libstub/lib-ksyms.o

We do not want to follow the default linking rules for static libraries
built under libstub/ so using subdir-y instead of obj-y is the correct
way to descend into this directory.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
[ardb: update commit log to clarify that arm64 deviates in this respect]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200305055047.6097-1-masahiroy@kernel.org
Link: https://lore.kernel.org/r/20200308080859.21568-23-ardb@kernel.org

drivers/firmware/efi/Makefile

@@ -20,7 +20,7 @@ obj-$(CONFIG_EFI_VARS_PSTORE)		+= efi-pstore.o
 obj-$(CONFIG_UEFI_CPER)			+= cper.o
 obj-$(CONFIG_EFI_RUNTIME_MAP)		+= runtime-map.o
 obj-$(CONFIG_EFI_RUNTIME_WRAPPERS)	+= runtime-wrappers.o
-obj-$(CONFIG_EFI_STUB)			+= libstub/
+subdir-$(CONFIG_EFI_STUB)		+= libstub
 obj-$(CONFIG_EFI_FAKE_MEMMAP)		+= fake_map.o
 obj-$(CONFIG_EFI_BOOTLOADER_CONTROL)	+= efibc.o
 obj-$(CONFIG_EFI_TEST)			+= test/