scripts: fix relpath.sh output when $prefix is set in environment

scripts: fix relpath.sh output when $prefix is set in environment

When relpath.sh is called from install target with prefix set, eg
"make install DESTDIR=/tmp/dpdk-root prefix=/usr", the prefix
from the environment leaks to relpath.sh internal helper variable and
causes incorrect symlinks to be generated in sdk $(targetdir):

   include -> /usr../../../include/dpdk
   lib -> /usr../../../lib

Initialize the local variable to empty to avoid side-effects from
environment.

Fixes: 6b62a72a70d0 ("mk: install a standard cutomizable tree")

Signed-off-by: Panu Matilainen <pmatilai@redhat.com>