selftests: Add install target

This adds make install support to selftests. The basic usage is:

$ cd tools/testing/selftests
$ make install

That installs into tools/testing/selftests/install, which can then be
copied where ever necessary.

The install destination is also configurable using eg:

$ INSTALL_PATH=/mnt/selftests make install

The implementation uses two targets in the child makefiles. The first
"install" is expected to install all files into $(INSTALL_PATH).

The second, "emit_tests", is expected to emit the test instructions (ie.
bash script) on stdout. Separating this from install means the child
makefiles need no knowledge of the location of the test script.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>