python-pyyaml: new package

[Thomas:
  - use 'select' rather than 'depends on' for the yaml library
    dependency
  - add hash file.
  - add upstream URL in Config.in help text
  - rework commit title.]

Signed-off-by: Geoffrey Ragot <geoffrey.ragot@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>