kbuild: list sub-directories in ./Kbuild

Use the ordinary obj-y syntax to list subdirectories.

Note1:
Previously, the link order of lib-y depended on CONFIG_MODULES; lib-y
was linked before drivers-y when CONFIG_MODULES=y, otherwise after
drivers-y. This was a bug of commit 7273ad2b08 ("kbuild: link lib-y
objects to vmlinux forcibly when CONFIG_MODULES=y"), but it was not a
big deal after all. Now, all objects listed in lib-y are linked last,
irrespective of CONFIG_MODULES.

Note2:
Finally, the single target build in arch/*/lib/ works correctly. There was
a bug report about this. [1]

  $ make ARCH=arm arch/arm/lib/findbit.o
    CALL    scripts/checksyscalls.sh
    AS      arch/arm/lib/findbit.o

[1]: https://lore.kernel.org/linux-kbuild/YvUQOwL6lD4%2F5%2FU6@shell.armlinux.org.uk/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

Kbuild

@@ -72,3 +72,27 @@ $(atomic-checks): $(obj)/.checked-%: include/linux/atomic/%  FORCE
 PHONY += prepare
 prepare: $(offsets-file) missing-syscalls $(atomic-checks)
 	@:
+
+# Ordinary directory descending
+# ---------------------------------------------------------------------------
+
+obj-y			+= init/
+obj-y			+= usr/
+obj-y			+= arch/$(SRCARCH)/
+obj-y			+= $(ARCH_CORE)
+obj-y			+= kernel/
+obj-y			+= certs/
+obj-y			+= mm/
+obj-y			+= fs/
+obj-y			+= ipc/
+obj-y			+= security/
+obj-y			+= crypto/
+obj-$(CONFIG_BLOCK)	+= block/
+obj-$(CONFIG_IO_URING)	+= io_uring/
+obj-y			+= $(ARCH_LIB)
+obj-y			+= drivers/
+obj-y			+= sound/
+obj-$(CONFIG_SAMPLES)	+= samples/
+obj-$(CONFIG_NET)	+= net/
+obj-y			+= virt/
+obj-y			+= $(ARCH_DRIVERS)