omniorb: force python interperter

Omniorb build-system requires python2 interpreter.

By default, it looks for python program, which may fallback on the system
python interpreter in case python is disabled and python3 is enabled.

So, this patch enforces the python interpreter to python2 built by
Buildroot.

Fixes:
  http://autobuild.buildroot.org/results/b4f/b4f3a2602ba0224ac3253c4bf6ed87ec045df772/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>