selftests: add headers_install to lib.mk

If the kernel headers aren't installed we can't build all the tests.
Add a new make target rule 'khdr' in the file lib.mk to generate the
kernel headers and that gets include for every test-dir Makefile that
includes lib.mk If the testdir in turn have its own sub-dirs the
top_srcdir needs to be set to the linux-rootdir to be able to generate
the kernel headers.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Fathi Boudra <fathi.boudra@linaro.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index 17ab366..0a8e758 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk
@@ -16,8 +16,20 @@
 TEST_GEN_PROGS_EXTENDED := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_PROGS_EXTENDED))
 TEST_GEN_FILES := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_FILES))
 
+top_srcdir ?= ../../../..
+include $(top_srcdir)/scripts/subarch.include
+ARCH		?= $(SUBARCH)
+
 all: $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES)
 
+.PHONY: khdr
+khdr:
+	make ARCH=$(ARCH) -C $(top_srcdir) headers_install
+
+ifdef KSFT_KHDR_INSTALL
+$(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES):| khdr
+endif
+
 .ONESHELL:
 define RUN_TEST_PRINT_RESULT
 	TEST_HDR_MSG="selftests: "`basename $$PWD`:" $$BASENAME_TEST";	\