package/gnuradio: bump to 3.8.1.0

package/gnuradio: bump to 3.8.1.0

- use github url
- remove all patches (applied in mainline)
- add DGR_PYTHON_DIR option to force install in site-packages instead of
  dist-packages

Fixes:

  http://autobuild.buildroot.net/results/ff29b01f41c7d2e139a319aac1406dc00185fcae/
  http://autobuild.buildroot.net/results/cae9ad5b746bfd728688ab655c23087a6f70fd50/
  http://autobuild.buildroot.net/results/333fd49196d71ea68f64ad046c38e480593f127e/

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>