target-finalize: fix calling ldconfig

On some systems (e.g. Fedora 20), the build breaks when parsing include
directives in our /etc/ld.so.conf, with error messages as thus:

    /sbin/ldconfig: need absolute file name for configuration file
    when using -r

So, enforce the path to the ld.so.conf file to point to our own, in the
target/ directory.

Reported-by: Al West <al.west@v-nova.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>