envsetup.sh: Make it work with zsh and ubuntu 14.04+

When sources from zsh BASH_SOURCE may not be available
lets use variable alteration to achieve what we want
and that works when bash script is sourced from zsh

Signed-off-by: Khem Raj <raj.khem@gmail.com>
use source insread of . we are using a bash script anyway

Signed-off-by: Khem Raj <raj.khem@gmail.com>