python-numpy: new package

This patch add NumPy package for python.

Since Fortran support has been deprecated in Buildroot since the 2013.11
release, and because most of the external toolchains do not provide a
Fortran compiler, it is necessary to explicity disable Fortran compiler
to avoid catching the one from the host system if any.

We also need to fill a site.cfg file to tell NumPy build-system where
it should looking for BLAS and LAPACK libraries.

Some packages may include headers provided by python-numpy package, so
python-numpy is installed to the staging directory.

[Thomas:
 - add dependency in Config.in to take into account fenv.h
   requirement, suggested by Yann E. Morin.
 - modified to use the staging installation logic in the python
   package infrastructure.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>