python: Fix ptest not working problems

python: Fix ptest not working problems

The run-ptest contained in python uses a Makefile. The path of the
Makefile is changed to ${libdir}/python/ptest in do_install_ptest.
However, the directory is wrong when the project is configured with
"--enable-multilib=lib32"

In addition, do_install_ptest is defined before "inherit ptest", so
it is overriden by the one in ptest.bbclass. do_install_ptest is
moved down.

(From OE-Core rev: 0eb947454e1c92467283e6f1adeca67c7c57698b)

Signed-off-by: Zhangle Yang <zhangle.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>