zephyr-env.sh: Fixed sourcing the project environment file from zsh

It will possible to source the project environment file from
both zsh and bash.

Change-Id: Ib6cbf4259378433bcf28a042898b69bb4b6b2a82
Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>

zephyr-env.sh

@@ -1,5 +1,5 @@
 
-if [ "X$(basename -- "$0")" == "Xzephyr-env.sh" ]; then
+if [ "X$(basename -z -- "$0")" "==" "Xzephyr-env.sh" ]; then
     echo "Source this file (do NOT execute it!) to set the Zephyr Kernel environment."
     exit
 fi