The existing logfile is simply placed in the current directory. Since the test
changes cwd to BUILDDIR, the symlink to the log can be placed in an invalid
directory. We also see trackbacks if the symlink is invalid.
Improve things by:
* Placing logs in LOG_DIR (or BUILDDIR if unset).
* Using a full path to the log meaning the log and link are placed in the same directory.
* Using lexists instead of exists so invalid symlinks are handled correctly.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
