selftests: drop khdr make target

Drop the "khdr" make target as it fails when the build directory is a
sub-directory of the source tree. Rely on the "headers_install"
target to have been run first instead.

For example, here's a typical error this patch is addressing:

$ make O=build -j32 kselftest-gen_tar
make[1]: Entering directory '/home/kernelci/linux/build'
make --no-builtin-rules INSTALL_HDR_PATH=/home/kernelci/linux/build/usr \
ARCH=x86 -C ../../.. headers_install
make[3]: Entering directory '/home/kernelci/linux'
Makefile:1022: ../scripts/Makefile.extrawarn: No such file or directory

The source directory is determined in the top-level Makefile as ".."
relatively to the "build" directory, but then the kselftest Makefile
switches to "-C ../../.." so "../scripts" then points one level higher
than the source tree e.g. "linux/../scripts" - which fails obviously.
There is no other use-case in the kernel tree where a sub-directory
Makefile tries to call a top-level make target, and it appears this
isn't really a valid thing to do.

Signed-off-by: Guillaume Tucker <[e-mail]>
Tested-by: Anders Roxell <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 # all isn't the first target in the file.
 .DEFAULT_GOAL := all
 
-# Install headers here once for all tests. KSFT_KHDR_INSTALL_DONE
-# is used to avoid running headers_install from lib.mk.
-# Invoke headers install with --no-builtin-rules to avoid circular
-# dependency in "make kselftest" case. In this case, second level
-# make inherits builtin-rules which will use the rule generate
-# Makefile.o and runs into
-# "Circular Makefile.o <- prepare dependency dropped."
-# and headers_install fails and test compile fails.
-#
-# O= KBUILD_OUTPUT cases don't run into this error, since main Makefile
-# invokes them as sub-makes and --no-builtin-rules is not necessary,
-# but doesn't cause any failures. Keep it simple and use the same
-# flags in both cases.
-# Local build cases: "make kselftest", "make -C" - headers are installed
-# in the default INSTALL_HDR_PATH usr/include.
-khdr:
-ifeq (1,$(DEFAULT_INSTALL_HDR_PATH))
-	$(MAKE) --no-builtin-rules ARCH=$(ARCH) -C $(top_srcdir) headers_install
-else
-	$(MAKE) --no-builtin-rules INSTALL_HDR_PATH=$(abs_objtree)/usr \
-		ARCH=$(ARCH) -C $(top_srcdir) headers_install
-endif
-
-all: khdr
+all:
 	@ret=1;							\
 	for TARGET in $(TARGETS); do				\
 		BUILD_TARGET=$$BUILD/$$TARGET;			\
···
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean;\
 	done;
 
-.PHONY: khdr all run_tests hotplug run_hotplug clean_hotplug run_pstore_crash install clean gen_tar
+.PHONY: all run_tests hotplug run_hotplug clean_hotplug run_pstore_crash install clean gen_tar