Add a nicer trace prompt for $PS4

Since we are standardizing on bash for shell scripts, we may as well
tell it to use a trace prompt that is vastly more useful for debugging.

This patchset changes $PS4 to prepend the script name and line number to
every line output in a debug trace.

Change-Id: I4ec5c0253295724a9580f86fd30082acd89d155d
Merge "Add a nicer trace prompt for $PS4"