build: create relative symlinks for PMDs in libdir

build: create relative symlinks for PMDs in libdir

Add -r option to ln, otherwise the link will be absolute and contain
the build path and break packaging among other things:

lrwxrwxrwx 1 bluca bluca     99 Sep 11 22:17 librte_mempool_dpaa.so.1.1
  -> /home/bluca/git/dpdk/testt4//usr/local/lib/x86_64-linux-gnu/dpdk/
     drivers/librte_mempool_dpaa.so.1.1

With -r:

lrwxrwxrwx 1 bluca bluca     35 Sep 12 18:13 librte_pmd_zlib.so.1.1
  -> dpdk/drivers/librte_pmd_zlib.so.1.1

Fixes: ed4d43d73e2b ("build: symlink drivers to library directory")
Cc: stable@dpdk.org
Signed-off-by: Luca Boccassi <bluca@debian.org>
Tested-by: Timothy Redaelli <tredaelli@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>