kbuild: fix building single targets with make O=.. single-target

This fixes single targets build so it now works relaiably in
following cases:
- build with mixed kernel source and output files (make single-target)
- build with separate output directory (make O=.. single-target)
- external module with mixed kernel source and output files
  (make M='pwd' single-target)
- external module with separate kernel source and output files
  (make O=.. M='pwd' single-target)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1275,40 +1275,43 @@ kernelversion:
 
 # Single targets
 # ---------------------------------------------------------------------------
-# The directory part is taken from first prerequisite, so this
-# works even with external modules
+# Single targets are compatible with:
+# - build whith mixed source and output
+# - build with separate output dir 'make O=...'
+# - external modules
+#
+#  target-dir => where to store outputfile
+#  build-dir  => directory in kernel source tree to use
+
+ifeq ($(KBUILD_EXTMOD),)
+        build-dir  = $(dir $@)
+        target-dir = $(dir $@)
+else
+        zap-slash=$(filter-out .,$(patsubst %/,%,$(dir $@)))
+        build-dir  = $(KBUILD_EXTMOD)$(if $(zap-slash),/$(zap-slash))
+        target-dir = $(if $(KBUILD_EXTMOD),$(dir $<),$(dir $@))
+endif
+
 %.s: %.c prepare scripts FORCE
-	$(Q)$(MAKE) $(build)=$(dir $<) $(dir $<)$(notdir $@)
+	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 %.i: %.c prepare scripts FORCE
-	$(Q)$(MAKE) $(build)=$(dir $<) $(dir $<)$(notdir $@)
+	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 %.o: %.c prepare scripts FORCE
-	$(Q)$(MAKE) $(build)=$(dir $<) $(dir $<)$(notdir $@)
+	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 %.lst: %.c prepare scripts FORCE
-	$(Q)$(MAKE) $(build)=$(dir $<) $(dir $<)$(notdir $@)
+	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 %.s: %.S prepare scripts FORCE
-	$(Q)$(MAKE) $(build)=$(dir $<) $(dir $<)$(notdir $@)
+	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 %.o: %.S prepare scripts FORCE
-	$(Q)$(MAKE) $(build)=$(dir $<) $(dir $<)$(notdir $@)
+	$(Q)$(MAKE) $(build)=$(build-dir) $(target-dir)$(notdir $@)
 
-# For external modules we shall include any directory of the target,
-# but usual case there is no directory part.
-# make M=`pwd` module.o     => $(dir $@)=./
-# make M=`pwd` foo/module.o => $(dir $@)=foo/
-# make M=`pwd` /            => $(dir $@)=/
- 
-ifeq ($(KBUILD_EXTMOD),)
-        target-dir = $(@D)
-else
-        zap-slash=$(filter-out .,$(patsubst %/,%,$(dir $@)))
-        target-dir = $(KBUILD_EXTMOD)$(if $(zap-slash),/$(zap-slash))
-endif
-
-/ %/:      scripts prepare FORCE
+# Modules
+/ %/: prepare scripts FORCE
 	$(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1) \
-	$(build)=$(target-dir)
-%.ko: scripts FORCE
+	$(build)=$(build-dir)
+%.ko: prepare scripts FORCE
 	$(Q)$(MAKE) KBUILD_MODULES=$(if $(CONFIG_MODULES),1)   \
-	$(build)=$(target-dir) $(@:.ko=.o)
+	$(build)=$(build-dir) $(@:.ko=.o)
 	$(Q)$(MAKE) -rR -f $(srctree)/scripts/Makefile.modpost
 
 # FIXME Should go into a make.lib or something