oe-init-buildenv/base: Relax python version checks in favour of HOSTTOOLS manipulation

oe-init-buildenv/base: Relax python version checks in favour of HOSTTOOLS manipulation

Several distros are now shipping "python" as python v3 contra to the original
python guidelines. This causes users confusion/pain in trying to use our tools.

We can just force "python" to "python2" within HOSTTOOLS to avoid this issue
and hide the complexity from the user.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>