Waf typically uses `python` as the intepretter but inside a task this
does not exist. Typically this is solved by patching waf (see the
glmark2 recipe) but not all versionf of Waf support Python 3 so we can't
assume a specific interpretter.
Instead, create a new variable WAF_PYTHON for the correct interpretter,
and default this to `python3`. If the user has a recipe that needs
Python 2 then this can be changed in the recipe.
Signed-off-by: Ross Burton <ross.burton@arm.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
