kbuild: Don't reset timestamps in include/generated if not needed

Use filechk to generate asm-offsets.h and bounds.h. Based on a patch by
Valdis Kletnieks.
Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Acked-By: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Kbuild

@@ -13,8 +13,9 @@ define sed-y
 	s:->::; p;}"
 endef
 
-quiet_cmd_offsets = GEN     $@
-define cmd_offsets
+# Use filechk to avoid rebuilds when a header changes, but the resulting file
+# does not
+define filechk_offsets
 	(set -e; \
 	 echo "#ifndef $2"; \
 	 echo "#define $2"; \
@@ -24,9 +25,9 @@ define cmd_offsets
 	 echo " * This file was generated by Kbuild"; \
 	 echo " */"; \
 	 echo ""; \
-	 sed -ne $(sed-y) $<; \
+	 sed -ne $(sed-y); \
 	 echo ""; \
-	 echo "#endif" ) > $@
+	 echo "#endif" )
 endef
 
 #####
@@ -42,9 +43,8 @@ kernel/bounds.s: kernel/bounds.c FORCE
 	$(Q)mkdir -p $(dir $@)
 	$(call if_changed_dep,cc_s_c)
 
-$(obj)/$(bounds-file): kernel/bounds.s Kbuild
-	$(Q)mkdir -p $(dir $@)
-	$(call cmd,offsets,__LINUX_BOUNDS_H__)
+$(obj)/$(bounds-file): kernel/bounds.s FORCE
+	$(call filechk,offsets,__LINUX_BOUNDS_H__)
 
 #####
 # 2) Generate asm-offsets.h
@@ -62,8 +62,8 @@ arch/$(SRCARCH)/kernel/asm-offsets.s: arch/$(SRCARCH)/kernel/asm-offsets.c \
 	$(Q)mkdir -p $(dir $@)
 	$(call if_changed_dep,cc_s_c)
 
-$(obj)/$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s Kbuild
-	$(call cmd,offsets,__ASM_OFFSETS_H__)
+$(obj)/$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s FORCE
+	$(call filechk,offsets,__ASM_OFFSETS_H__)
 
 #####
 # 3) Check for missing system calls