selftests: fix install target to use default install path

Install target fails when INSTALL_PATH is undefined. Fix install target
to use "output_dir/install as the default install location. "output_dir"
is either the root of selftests directory under kernel source tree or
output directory specified by O= or KBUILD_OUTPUT.

e.g:
make -C tools/testing/selftests install
<installs under tools/testing/selftests/install>

make O=/tmp/kselftest -C tools/testing/selftests install
<installs under /tmp/kselftest/install>

export KBUILD_OUTPUT=/tmp/kselftest
make -C tools/testing/selftests install
<installs under /tmp/kselftest/install>

In addition, add "all" target as dependency to "install" to build and
install using a single command.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>