scripts: fix build in a symlinked directory

scripts: fix build in a symlinked directory

The script relpath.sh returns the relative path of the first directory
from the second directory. It is used to generate relative symlinks,
which can be useful if the build directory is embedded in the dpdk
directory: the whole dpdk can be moved without breaking the links,
which is helpful for an installation.

In case the build directory is a symlink, the script was not generating
the proper relative path. Fix this by calling "readlink -f" on the
arguments.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Tested-by: Simon Kagstrom <simon.kagstrom@netinsight.net>