kbuild: replace KBUILD_SRCTREE with boolean building_out_of_srctree

Commit 25b146c5b8 ("kbuild: allow Kbuild to start from any directory")
deprecated KBUILD_SRCTREE.

It is only used in tools/testing/selftest/ to distinguish out-of-tree
build. Replace it with a new boolean flag, building_out_of_srctree.

I also replaced the conditional ($(srctree),.) because the next commit
will allow an absolute path to be used for $(srctree) even when building
in the source tree.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.build

@@ -513,7 +513,7 @@ existing-targets := $(wildcard $(sort $(targets)))
 
 -include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
 
-ifneq ($(srctree),.)
+ifdef building_out_of_srctree
 # Create directories for object files if they do not exist
 obj-dirs := $(sort $(obj) $(patsubst %/,%, $(dir $(targets))))
 # If targets exist, their directories apparently exist. Skip mkdir.

scripts/Makefile.host

@@ -69,7 +69,7 @@ _hostcxx_flags = $(KBUILD_HOSTCXXFLAGS) $(HOST_EXTRACXXFLAGS) \
 
 # $(objtree)/$(obj) for including generated headers from checkin source files
 ifeq ($(KBUILD_EXTMOD),)
-ifneq ($(srctree),.)
+ifdef building_out_of_srctree
 _hostc_flags   += -I $(objtree)/$(obj)
 _hostcxx_flags += -I $(objtree)/$(obj)
 endif

scripts/Makefile.lib

@@ -154,7 +154,7 @@ endif
 # $(srctree)/$(src) for including checkin headers from generated source files
 # $(objtree)/$(obj) for including generated headers from checkin source files
 ifeq ($(KBUILD_EXTMOD),)
-ifneq ($(srctree),.)
+ifdef building_out_of_srctree
 _c_flags   += -I $(srctree)/$(src) -I $(objtree)/$(obj)
 _a_flags   += -I $(srctree)/$(src) -I $(objtree)/$(obj)
 _cpp_flags += -I $(srctree)/$(src) -I $(objtree)/$(obj)

scripts/Makefile.modbuiltin

@@ -15,7 +15,7 @@ include include/config/tristate.conf
 
 include scripts/Kbuild.include
 
-ifneq ($(srctree),.)
+ifdef building_out_of_srctree
 # Create output directory if not already present
 _dummy := $(shell [ -d $(obj) ] || mkdir -p $(obj))
 endif

scripts/gdb/linux/Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
-ifneq ($(srctree),.)
+ifdef building_out_of_srctree
 
 symlinks := $(patsubst $(srctree)/$(src)/%,%,$(wildcard $(srctree)/$(src)/*.py))