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

Use filechk to generate asm-offsets.h and generic-asm-offsets.h.
Based on a patch by Valdis Kletnieks.
Reported-by: NValdis Kletnieks <Valdis.Kletnieks@vt.edu>
Acked-by: NValdis Kletnieks <Valdis.Kletnieks@vt.edu>
Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NMichal Marek <mmarek@suse.cz>
[ imported from Linux Kernel, commit 70a4fd6c56d0,
  with adjustment for U-Boot ]
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

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 @@ lib/asm-offsets.s: lib/asm-offsets.c FORCE
 	$(Q)mkdir -p $(dir $@)
 	$(call if_changed_dep,cc_s_c)
 
-$(obj)/$(generic-offsets-file): lib/asm-offsets.s Kbuild
-	$(Q)mkdir -p $(dir $@)
-	$(call cmd,offsets,__GENERIC_ASM_OFFSETS_H__)
+$(obj)/$(generic-offsets-file): lib/asm-offsets.s FORCE
+	$(call filechk,offsets,__GENERIC_ASM_OFFSETS_H__)
 
 #####
 # 2) Generate asm-offsets.h
@@ -65,5 +65,5 @@ arch/$(ARCH)/lib/asm-offsets.s: arch/$(ARCH)/lib/asm-offsets.c FORCE
 	$(Q)mkdir -p $(dir $@)
 	$(call if_changed_dep,cc_s_c)
 
-$(obj)/$(offsets-file): arch/$(ARCH)/lib/asm-offsets.s Kbuild
-	$(call cmd,offsets,__ASM_OFFSETS_H__)
+$(obj)/$(offsets-file): arch/$(ARCH)/lib/asm-offsets.s FORCE
+	$(call filechk,offsets,__ASM_OFFSETS_H__)