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>