uboot-imx

tools: binman: Use /usr/bin/env to find python executable

Some OS (all BSD and probably others) do not have python in /usr/bin
but in another directory.
It is a common usage to use /usr/bin/env python as shebang for python
scripts so use this for binman.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>