selftests: gen_kselftest_tar.sh: Do not clobber kselftest/

The default installation location for gen_kselftest_tar.sh was still
"kselftest/" which collides with the existing directory. Instead, this
moves the installation target into "kselftest_install/kselftest/" and
adjusts the tar creation accordingly. This also adjusts indentation and
logic to be consistent.

Fixes: 42d46e57ec97 ("selftests: Extract single-test shell logic from lib.mk")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests: Add kselftest-all and kselftest-install targets

Add kselftest-all target to build tests from the top level
Makefile. This is to simplify kselftest use-cases for CI and
distributions where build and test systems are different.

Current kselftest target builds and runs tests on a development
system which is a developer use-case.

Add kselftest-install target to install tests from the top level
Makefile. This is to simplify kselftest use-cases for CI and
distributions where build and test systems are different.

This change addresses requests from developers and testers to add
support for installing kselftest from the main Makefile.

In addition, make the install directory the same when install is
run using "make kselftest-install" or by running kselftest_install.sh.
Also fix the INSTALL_PATH variable conflict between main Makefile and
selftests Makefile.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

selftests: kselftest_install.sh: Add SPDX license identifier

Replace GPL license statement with SPDX GPL-2.0 license identifier.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

selftests: Add kselftest install tool

kselftest_install.sh tool installs selftests in default location
which is tools/testing/selftests/kselftest or an user specified
location. This tool invokes back-end selftests install target with
the install location.

Usage:
cd tools/testing/selftests
./kselftest_install.sh [ install_dir ]

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>