tools: allow to override python

Not force to use python from PATH. Issue was noted when building with
Yocto, because python from the distro is always taken instead of
python-native built during Yocto process.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>