cicd: a series of small improvements

.github/scripts/cross.sh

@@ -1,9 +1,16 @@
 #!/bin/bash
 
-set -x
+# set -x
 set -e
 
-if [ "$CROSS" != 1 ]; then
+if [ "$BUILD_CMD" != "cross" ]; then
+    echo "cross.sh - is a helper to assist only in cross compiling environments" >&2
+    echo "To use this tool set the BUILD_CMD env var to the \"cross\" value" >&2
+    exit 111
+fi
+
+if [ -z "$CROSS_IMAGE" ]; then
+    echo "The CROSS_IMAGE env var should be provided" >&2
     exit 111
 fi