Waf's dependency on Python3

[Jonathan Brandmeyer]

I attempted to follow the directions in rtems-libbsd's README.md and
run into the following error: "Could not create the directory ///h",
right after configuring the build.  On a wild guess I tried again
using python3 as the interpreter explicitly and that succeeded.

My host is Debian Stretch, AMD64.
/usr/bin/python is 2.7.13
/usr/bin/python3 is 3.5.3
Tested on rtems-libbsd current master
(5432c6bed37fa26a27c2730e34343d4c507902a9 as of this writing).

Its not entirely clear to me if waf is supposed to be dual-mode
compatible right now or not.  Maybe the waf shebang line should be
updated?

Thanks,
Jonathan