selftests: Fix test errors related to lib.mk khdr target

Commit b2d35fa5fc ("selftests: add headers_install to lib.mk") added
khdr target to run headers_install target from the main Makefile. The
logic uses KSFT_KHDR_INSTALL and top_srcdir as controls to initialize
variables and include files to run headers_install from the top level
Makefile. There are a few problems with this logic.

1. Exposes top_srcdir to all tests
2. Common logic impacts all tests
3. Uses KSFT_KHDR_INSTALL, top_srcdir, and khdr in an adhoc way. Tests
   add "khdr" dependency in their Makefiles to TEST_PROGS_EXTENDED in
   some cases, and STATIC_LIBS in other cases. This makes this framework
   confusing to use.

The common logic that runs for all tests even when KSFT_KHDR_INSTALL
isn't defined by the test. top_srcdir is initialized to a default value
when test doesn't initialize it. It works for all tests without a sub-dir
structure and tests with sub-dir structure fail to build.

e.g: make -C sparc64/drivers/ or make -C drivers/dma-buf

../../lib.mk:20: ../../../../scripts/subarch.include: No such file or directory
make: *** No rule to make target '../../../../scripts/subarch.include'.  Stop.

There is no reason to require all tests to define top_srcdir and there is
no need to require tests to add khdr dependency using adhoc changes to
TEST_* and other variables.

Fix it with a consistent use of KSFT_KHDR_INSTALL and top_srcdir from tests
that have the dependency on headers_install.

Change common logic to include khdr target define and "all" target with
dependency on khdr when KSFT_KHDR_INSTALL is defined.

Only tests that have dependency on headers_install have to define just
the KSFT_KHDR_INSTALL, and top_srcdir variables and there is no need to
specify khdr dependency in the test Makefiles.

Fixes: b2d35fa5fc ("selftests: add headers_install to lib.mk")
Cc: stable@vger.kernel.org
Signed-off-by: Shuah Khan <shuah@kernel.org>
Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
Reviewed-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>

tools/testing/selftests/gpio/Makefile

@@ -16,8 +16,6 @@ TEST_PROGS_EXTENDED := gpio-mockup-chardev
 GPIODIR := $(realpath ../../../gpio)
 GPIOOBJ := gpio-utils.o
 
-include ../lib.mk
-
 all: $(TEST_PROGS_EXTENDED)
 
 override define CLEAN
@@ -25,7 +23,9 @@ override define CLEAN
 	$(MAKE) -C $(GPIODIR) OUTPUT=$(GPIODIR)/ clean
 endef
 
-$(TEST_PROGS_EXTENDED):| khdr
+KSFT_KHDR_INSTALL := 1
+include ../lib.mk
+
 $(TEST_PROGS_EXTENDED): $(GPIODIR)/$(GPIOOBJ)
 
 $(GPIODIR)/$(GPIOOBJ):