kbuild: error out if targets prefixed with '__' are directly run

Some targets are internal-use only.

It is tedious to care about "what if __build_one_by_one is contained
in $(MAKECMDGOALS)?" etc.

Prefix internal targets with double underscores. Stop parsing Makefile
if they are directly run.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -11,9 +11,12 @@ NAME = Kleptomaniac Octopus
 # Comments in this file are targeted only to the developer, do not
 # expect to learn how to build the kernel reading this file.
 
+$(if $(filter __%, $(MAKECMDGOALS)), \
+	$(error targets prefixed with '__' are only for internal use))
+
 # That's our default target when none is given on the command line
-PHONY := _all
-_all:
+PHONY := __all
+__all:
 
 # We are using a recursive build, so we need to do a little thinking
 # to get the ordering right.
@@ -172,13 +175,13 @@ export sub_make_done := 1
 
 ifeq ($(need-sub-make),1)
 
-PHONY += $(MAKECMDGOALS) sub-make
+PHONY += $(MAKECMDGOALS) __sub-make
 
-$(filter-out _all sub-make $(this-makefile), $(MAKECMDGOALS)) _all: sub-make
+$(filter-out $(this-makefile), $(MAKECMDGOALS)) __all: __sub-make
 	@:
 
 # Invoke a second make in the output directory, passing relevant variables
-sub-make:
+__sub-make:
 	$(Q)$(MAKE) -C $(abs_objtree) -f $(abs_srctree)/Makefile $(MAKECMDGOALS)
 
 endif # need-sub-make
@@ -323,7 +326,7 @@ ifdef mixed-build
 
 PHONY += $(MAKECMDGOALS) __build_one_by_one
 
-$(filter-out __build_one_by_one, $(MAKECMDGOALS)): __build_one_by_one
+$(MAKECMDGOALS): __build_one_by_one
 	@:
 
 __build_one_by_one:
@@ -598,12 +601,12 @@ else #!config-build
 # targets and others. In general all targets except *config targets.
 
 # If building an external module we do not care about the all: rule
-# but instead _all depend on modules
+# but instead __all depend on modules
 PHONY += all
 ifeq ($(KBUILD_EXTMOD),)
-_all: all
+__all: all
 else
-_all: modules
+__all: modules
 endif
 
 # Decide whether to build built-in, modular, or both.
@@ -625,7 +628,7 @@ endif
 # in addition to whatever we do anyway.
 # Just "make" or "make all" shall build modules as well
 
-ifneq ($(filter all _all modules nsdeps,$(MAKECMDGOALS)),)
+ifneq ($(filter all modules nsdeps,$(MAKECMDGOALS)),)
   KBUILD_MODULES := 1
 endif