Documentation: use subdir-y to avoid unnecessary built-in.o files

Change the Documentation makefiles from obj-m to subdir-y
to avoid generating unnecessary built-in.o files since nothing
in Documentation/ is ever linked in to vmlinux.

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/networking/Makefile

@@ -1,7 +1 @@
-# kbuild trick to avoid linker error. Can be omitted if a module is built.
-obj- := dummy.o
-
-# Tell kbuild to always build the programs
-always := $(hostprogs-y)
-
-obj-m := timestamping/
+subdir-y := timestamping

Documentation/networking/timestamping/Makefile

@@ -1,6 +1,3 @@
-# kbuild trick to avoid linker error. Can be omitted if a module is built.
-obj- := dummy.o
-
 # List of programs to build
 hostprogs-y := timestamping hwtstamp_config