trace-cmd: use pkg-config instead of python-config

trace-cmd use python-config to find out which headers
and libraries should be used to link against the Python libraries.
By default, python-config returns paths that are inappropriate for
cross-compilation.

This patch replaces python-config with pkg-config as a workaround.

Add PYTHON_VERS to build trace-cmd with python2 or python3

Fixes:
http://autobuild.buildroot.net/results/980/980875810528ac1dee34b8c268d9b3c40b2e35ec/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>