kbuild: Restore build nr, improve vmlinux link

People relied on the build generation number to find out if they actually
booted the right kernel, only incrementing it on config changes isn't
enough for them. So this patch goes back to the old behavior, only done
right this time:

We now don't increment the generation number on every invocation of make, 
but on every link of vmlinux. At the same time, this patch fixes the
Makefile to only relink vmlinux if any of its prequisite object files
(or the command line) changed. Incrementing the version number
happens *after* we decided to relink vmlinux.

Makefile

@@ -159,14 +159,25 @@ export	CPPFLAGS CFLAGS CFLAGS_KERNEL AFLAGS AFLAGS_KERNEL
 
 export	NETWORKS DRIVERS LIBS HEAD LDFLAGS LINKFLAGS MAKEBOOT ASFLAGS
 
-# Build vmlinux / boot target
+# boot target
 # ---------------------------------------------------------------------------
 
 boot: vmlinux
 	@$(MAKE) CFLAGS="$(CFLAGS) $(CFLAGS_KERNEL)" AFLAGS="$(AFLAGS) $(AFLAGS_KERNEL)" -C arch/$(ARCH)/boot
 
-vmlinux: include/linux/version.h $(CONFIGURATION) linuxsubdirs
-	$(LD) $(LINKFLAGS) $(HEAD) $(INIT) \
+# Build vmlinux
+# ---------------------------------------------------------------------------
+
+#	This is a bit tricky: If we need to relink vmlinux, we want
+#	the version number incremented, which means recompile init/version.o
+#	and relink init/init.o. However, we cannot do this during the
+#       normal descending-into-subdirs phase, since at that time
+#       we cannot yet know if we will need to relink vmlinux.
+#	So we descend into init/ inside the rule for vmlinux again.
+
+vmlinux-objs := $(HEAD) $(INIT) $(CORE_FILES) $(LIBS) $(DRIVERS) $(NETWORKS)
+
+cmd_link_vmlinux = $(LD) $(LINKFLAGS) $(HEAD) $(INIT) \
 		--start-group \
 		$(CORE_FILES) \
 		$(LIBS) \
@@ -174,10 +185,33 @@ vmlinux: include/linux/version.h $(CONFIGURATION) linuxsubdirs
 		$(NETWORKS) \
 		--end-group \
 		-o vmlinux
+
+#	set -e makes the rule exit immediately on error
+
+define rule_link_vmlinux
+	set -e
+	echo Generating build number
+	. scripts/mkversion > .tmpversion
+	mv -f .tmpversion .version
+	$(MAKE) CFLAGS="$(CFLAGS) $(CFLAGS_KERNEL)" AFLAGS="$(AFLAGS) $(AFLAGS_KERNEL)" -C init
+	echo $(cmd_link_vmlinux)
+	$(cmd_link_vmlinux)
+	echo 'cmd_$@ := $(cmd_link_vmlinux)' > $(@D)/.$(@F).cmd
 	$(NM) vmlinux | grep -v '\(compiled\)\|\(\.o$$\)\|\( [aUw] \)\|\(\.\.ng$$\)\|\(LASH[RL]DI\)' | sort > System.map
+endef
+
+vmlinux: $(CONFIGURATION) $(vmlinux-objs) dummy
+	$(call if_changed_rule,link_vmlinux)
+
+#	The actual objects are generated when descending, make sure
+#	no implicit rule kicks in
+
+$(sort $(vmlinux-objs)): linuxsubdirs
+	@
 
 # 	Handle descending into subdirectories listed in $(SUBDIRS)
 
+.PHONY: linuxsubdirs
 linuxsubdirs: $(patsubst %, _dir_%, $(SUBDIRS))
 
 $(patsubst %, _dir_%, $(SUBDIRS)) : dummy include/linux/version.h include/config/MARKER
@@ -214,8 +248,6 @@ symlinks:
 include/config/MARKER: scripts/split-include include/linux/autoconf.h
 	scripts/split-include include/linux/autoconf.h include/config
 	@ touch include/config/MARKER
-	. scripts/mkversion > .tmpversion
-	@mv -f .tmpversion .version
 
 # Generate some files
 
@@ -470,3 +502,12 @@ backup: mrproper
 
 sums:
 	find . -type f -print | sort | xargs sum > .SUMS
+
+# FIXME Should go into a make.lib or something 
+# ---------------------------------------------------------------------------
+
+if_changed_rule = $(if $(strip $? \
+		               $(filter-out $(cmd_$(1)),$(cmd_$(@F)))\
+			       $(filter-out $(cmd_$(@F)),$(cmd_$(1)))),\
+	               @$(rule_$(1)))
+