gnuradio: break dependency chain by using 'depends on'

This commit fixes the following recursive dependency:

package/python/Config.in:5:error: recursive dependency detected!
package/python/Config.in:5:	symbol BR2_PACKAGE_PYTHON is selected by BR2_PACKAGE_GNURADIO_PYTHON
package/gnuradio/Config.in:46:	symbol BR2_PACKAGE_GNURADIO_PYTHON is selected by BR2_PACKAGE_GNURADIO_UTILS
package/gnuradio/Config.in:55:	symbol BR2_PACKAGE_GNURADIO_UTILS depends on BR2_PACKAGE_PYTHON_NUMPY_ARCH_SUPPORTS
package/python-numpy/Config.in:1:	symbol BR2_PACKAGE_PYTHON_NUMPY_ARCH_SUPPORTS depends on BR2_PACKAGE_PYTHON

by replacing the "select BR2_PACKAGE_PYTHON" by a "depends on
BR2_PACKAGE_PYTHON", and ditto for the BR2_PACKAGE_GNURADIO_PYTHON
selection.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>