Encode $(NO_PYTHON) consistently with other variables

We have several $(NO_*) variables used to disable optional features.
$(NO_PYTHON) is encoded as empty for "include Python support" and anything
else for "disable Python support".

However the other variables - $(NO_YAML) and $(NO_VALGRIND) - use 0 for
"include" and 1 for "disable".  Change $(NO_PYTHON) to work consistently
with the others.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>