bindings: python: fix out-of-tree build

bindings: python: fix out-of-tree build

Makefile.am delegates the build of the python extension to its
setup.py file, which references the extension .c files relative to the
source dir. This makes it impossible to build in a directory that is
different from the source directory (for example, for PC and ARM but
from the same source).

* Invoke setup.py from $(srcdir)
* Modify setup.py to pick up .c files relative from setup.py's own
  directory.

Signed-off-by: Joerg Faschingbauer <jf@faschingbauer.co.at>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>