package/environment-setup/environment-setup: add zsh

package/environment-setup/environment-setup: add zsh

environment-setup uses BASH_SOURCE which is bash specific. For other
shells, this variable is empty, leading to an error message and empty
SDK_PATH.
Zsh Uses $0. Unfortunately POSIX is not specifying how exactly $0
should behave when in sourced (or using special dot utility). So other
shell support have to be implemented in different manner.

Signed-off-by: Krzysztof Kanas <kkanas@fastmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>