doc: dev-tools: kselftest.rst: Update examples and paths

Update the installation commands and path details, detail the new
options available in the run_kselftests.sh script.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>