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>

Documentation/dev-tools/kselftest.rst

@@ -151,6 +151,29 @@ note some tests will require root privileges::
    $ cd kselftest
    $ ./run_kselftest.sh
 
+Packaging selftests
+===================
+
+In some cases packaging is desired, such as when tests need to run on a
+different system. To package selftests, run::
+
+   $ make -C tools/testing/selftests gen_tar
+
+This generates a tarball in the `INSTALL_PATH/kselftest-packages` directory. By
+default, `.gz` format is used. The tar format can be overridden by specifying
+a `FORMAT` make variable. Any value recognized by `tar's auto-compress`_ option
+is supported, such as::
+
+    $ make -C tools/testing/selftests gen_tar FORMAT=.xz
+
+`make gen_tar` invokes `make install` so you can use it to package a subset of
+tests by using variables specified in `Running a subset of selftests`_
+section::
+
+    $ make -C tools/testing/selftests gen_tar TARGETS="bpf" FORMAT=.xz
+
+.. _tar's auto-compress: https://www.gnu.org/software/tar/manual/html_node/gzip.html#auto_002dcompress
+
 Contributing new tests
 ======================