Add PS4 to show file/function/line in debug output

For something fairly simple, I went back-and-forward with this a bit.

Firstly, I realise calling readlink constantly sucks.  Due to the way
we call dib and source various files, you end up with the source-file
from "caller" being usually a very ugly path including levels of "../"
indirection.  Cleaning this up to something canonical is the only sane
way to present it.

Because we evaluate _ps4() from a sub-shell in the PS4 string, there's
no way for it to do something like build a global in-memory cache in
an associative array or similar.  It could write out a temp file or
some other side-band method, but the overheads of managing this don't
seem any different to just calling readlink.  If anyone can think of a
bash-hack around this that doesn't involve a fork() I'm interested.

We could potentially strip some of the leading paths in the assumption
you know what they are; but it gets complex when things are split
across /usr/bin & /usr/lib and external elements, etc.  I thought
about arbitrarily shortening it (e.g. just take last 20 characters)
which gives you enough of an idea of the file, but looks a bit ugly.
Or we could just leave the file-name out all together and assume the
function name is unique enough; this also seemed a bit ugly.

Obviously it's a matter of taste in the output.  It is certainly
wider, but it also adds a lot of information.  It also makes it fairly
clear where there are things we can make less verbose,
e.g. I1e39822f218dc0322e2490a770f3dc867a55802c disables tracing in
run-parts which is just noise.  There's a few other frequently used
loops that we could disable tracing for by default to benefit
signal:noise.

tl;dr : take a look at the logs.  I think it is a step in the right
direction of making the logs more usable for debugging.

Change-Id: I8054a3050415fcb527baeb7012bf133e5c864bf3

bin/disk-image-create

@@ -216,6 +216,17 @@ if [ "$CLEAR_ENV" = "1" -a "$HOME" != "" ]; then
   exec -c $0 "$@"
 fi
 
+
+# Display the current file/function/line in the debug output
+function _ps4 {
+    IFS=" " called=($(caller 0))
+    local f=$(readlink -f ${called[2]})
+    printf "%-80s " "$f:${called[1]}:${called[0]}"
+}
+export -f _ps4
+export PS4='+ $(_ps4):   '
+
+
 source $_LIB/img-defaults
 source $_LIB/common-functions
 source $_LIB/img-functions