Enabled buildbot_steps script to be sourced or ran directly by bash

buildbot/buildbot_steps.sh

@@ -480,3 +480,12 @@ function upload_release_ipxe_shasums() {
 
     return 0
 }
+
+if [[ ! "$(ps -o cmd -p $$ | tail -n 1 | tr -d '\n')" =~ "bash" ]]; then
+    >&2 echo "Shell not bash, exiting."
+    exit 1
+fi
+
+# This is a bash-specific hack to determine if the file is being sourced or
+# run as a script. It will only work in bash, not other shells.
+(return 0 2>/dev/null) || "$@"