Fix ldconfig call in target-finalize

Recent versions of ldconfig require the directory /var/lib/ldconfig,
otherwise they bail out with an error. Moreover, having a
/etc/ld.so.conf file also avoids a warning.

In addition to this, we remove the redirection of stderr, so that
errors remain visible to the user.

Thanks to Baruch Siach, Andy Gibbs and Darcy Watkins for investigating
the issue and proposing solutions.

This fixes the build on recent distributions such as Fedora 13 or
Gentoo.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>