kbuild: fix single target build for external module

Building a single target in an external module fails due to missing
.tmp_versions directory.

For example,

  $ make -C /lib/modules/$(uname -r)/build M=$PWD foo.o

will fail in the following way:

  CC [M]  /home/masahiro/foo/foo.o
/bin/sh: 1: cannot create /home/masahiro/foo/.tmp_versions/foo.mod: Directory nonexistent

This is because $(cmd_crmodverdir) is executed only before building
/, %/, %.ko single targets of external modules. Create .tmp_versions
in the 'prepare' target.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1551,9 +1551,6 @@ else # KBUILD_EXTMOD
 
 # We are always building modules
 KBUILD_MODULES := 1
-PHONY += crmodverdir
-crmodverdir:
-	$(cmd_crmodverdir)
 
 PHONY += $(objtree)/Module.symvers
 $(objtree)/Module.symvers:
@@ -1565,7 +1562,7 @@ $(objtree)/Module.symvers:
 
 module-dirs := $(addprefix _module_,$(KBUILD_EXTMOD))
 PHONY += $(module-dirs) modules
-$(module-dirs): crmodverdir $(objtree)/Module.symvers
+$(module-dirs): prepare $(objtree)/Module.symvers
 	$(Q)$(MAKE) $(build)=$(patsubst _module_%,%,$@)
 
 modules: $(module-dirs)
@@ -1606,7 +1603,8 @@ help:
 
 # Dummies...
 PHONY += prepare scripts
-prepare: ;
+prepare:
+	$(cmd_crmodverdir)
 scripts: ;
 endif # KBUILD_EXTMOD
 
@@ -1730,17 +1728,14 @@ endif
 
 # Modules
 /: prepare scripts FORCE
-	$(cmd_crmodverdir)
 	$(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1) \
 	$(build)=$(build-dir)
 # Make sure the latest headers are built for Documentation
 Documentation/ samples/: headers_install
 %/: prepare scripts FORCE
-	$(cmd_crmodverdir)
 	$(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1) \
 	$(build)=$(build-dir)
 %.ko: prepare scripts FORCE
-	$(cmd_crmodverdir)
 	$(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1)   \
 	$(build)=$(build-dir) $(@:.ko=.o)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost