openpgm: make its build system python2/python3 friendly

This patch fixes build issue when python3 is enabled, and host python2
interpreter is not available.

Fixes:
  http://autobuild.buildroot.net/results/01f/01f886a073439c0639ed93e596b68bcadf1a5824/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>