kbuild: fix building with redirected output.

Jan Altenberg <jan.altenberg@linutronix.de> reported that
building with redirected input like this failed:
make O=dir oldconfig bzImage < /dev/null

The problem were caused by a make silentoldconfig being
run before oldconfig and with a non-recent .config the build
failed because silentoldconfig requires non-redirected stdin.

Silentoldconfig was run as a side-effect of having the
top-level Makefile re-made by make.
Introducing an empty rule for the top-level Makefile
(and Kbuild.include) fixed the issue.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -108,6 +108,9 @@ endif
 PHONY := _all
 _all:
 
+# Cancel implicit rules on top Makefile
+$(CURDIR)/Makefile Makefile: ;
+
 ifneq ($(KBUILD_OUTPUT),)
 # Invoke a second make in the output directory, passing relevant variables
 # check that the output directory actually exists
@@ -121,7 +124,7 @@ $(if $(filter-out $(KBUILD_OUTPUT),$(shell /bin/pwd)),, \
 
 PHONY += $(MAKECMDGOALS) sub-make
 
-$(filter-out _all sub-make,$(MAKECMDGOALS)) _all: sub-make
+$(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
 	$(Q)@:
 
 sub-make: FORCE
@@ -291,7 +294,8 @@ export quiet Q KBUILD_VERBOSE
 # Look for make include files relative to root of kernel src
 MAKEFLAGS += --include-dir=$(srctree)
 
-# We need some generic definitions.
+# We need some generic definitions (do not try to remake the file).
+$(srctree)/scripts/Kbuild.include: ;
 include $(srctree)/scripts/Kbuild.include
 
 # Make variables (CC, etc...)
@@ -1560,9 +1564,6 @@ endif	# skip-makefile
 PHONY += FORCE
 FORCE:
 
-# Cancel implicit rules on top Makefile, `-rR' will apply to sub-makes.
-Makefile: ;
-
 # Declare the contents of the .PHONY variable as phony.  We keep that
 # information in a variable se we can use it in if_changed and friends.
 .PHONY: $(PHONY)