sstate: Fix the relative symlink replacement code

ant reported on irc that the sstate absolute to relative symlink creation
code wasn't working in klibc. He was correct although the level of breakage is
rather surprising since it only worked for one level of symlink (usr/include) with
everything else being broken.

The reason is probably that nothing really uses absolute paths, we use relative
paths where at all possible already. Nothing in the target sysroot should use
absolute paths for a start. In this regard, the klibc-dev package is broken and
needs fixing. It will currently break when building for one machine, then switching
to another of the same TUNE_PKGARCH and installing from sstate but that is a
separate issue.

This patch fixes the symlink creation code by firstly passing in the correct
value we need (where the symlink will end up) and seccondly, actually using it.

I've also tweaked the debug message to contain appropriate information and got
right of the double "//" value the existing code created in favour of the form
'./..' which looks neater.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>