tweak ps4 for readability

After staring at a bunch of logs, try to crisp up the ps4 output for
maximum readability.

This also adds PS4 to all calling scripts by having a common PS4 in
stackrc. It should make understanding when clean fails a bit more
straight forward.

Change-Id: Ia1f8dff5c1102c32c146a020a9f033c65d2c50de
diff --git a/stackrc b/stackrc
index 58146a4..887d4b3 100644
--- a/stackrc
+++ b/stackrc
@@ -14,6 +14,9 @@
 LC_ALL=C
 export LC_ALL
 
+# Make tracing more educational
+export PS4='+ ${BASH_SOURCE}:${FUNCNAME[0]}:L${LINENO}:   '
+
 # Find the other rc files
 RC_DIR=$(cd $(dirname "${BASH_SOURCE:-$0}") && pwd)