tools/genboardscfg.py: change shebang into /usr/bin/env python2

This tool only works on python 2 (python 2.6 or lator).

Change the shebang to make sure the script is run by python 2
and clearly say the supported version in the comment block.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>