selftests: introduce gen_tar Makefile target

The gen_kselftest_tar.sh always packages *all* selftests and doesn't
pass along any variables to `make install` to influence what should be
built. This can result in an early error on the command line ("Unknown
tarball format TARGETS=XXX"), or unexpected test failures as the
tarball contains tests people wanted to skip on purpose.

Since the makefile already contains all the logic, we can add a target
for packaging. Keep the default .gz target the script uses, and actually
extend the supported formats by using tar's autodetection.

To not break current workflows, keep the gen_kselftest_tar.sh script as
it is, with an added suggestion to use the makefile target instead.

Signed-off-by: Veronika Kabatova <vkabatov@redhat.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/gen_kselftest_tar.sh

@@ -49,6 +49,11 @@ main()
 	# directory
 	./kselftest_install.sh "$install_dir"
 	(cd "$install_work"; tar $copts "$dest"/kselftest${ext} $install_name)
+
+	# Don't put the message at the actual end as people may be parsing the
+	# "archive created" line in their scripts.
+	echo -e "\nConsider using 'make gen_tar' instead of this script\n"
+
 	echo "Kselftest archive kselftest${ext} created!"
 
 	# clean up top-level install work directory