mk: fix libs installation when installing sdk

mk: fix libs installation when installing sdk

The 'install-sdk' target creates an invalid symlink in the install
directory:

  # make install-sdk DESTDIR=/tmp/toto V=1
  [...]
  ln -snf $(/root/dpdk.org/buildtools/relpath.sh /tmp/toto/usr/local/lib \
            /tmp/toto/usr/local/share/dpdk/x86_64-native-linuxapp-gcc/) \
       /tmp/toto/usr/local/share/dpdk/x86_64-native-linuxapp-gcc/lib
  [...]
  # ls -l /tmp/toto/usr/local/share/dpdk/x86_64-native-linuxapp-gcc
  drwxr-xr-x 2 root root 4096 Dec 11 11:49 app
  lrwxrwxrwx 1 root root   21 Dec 11 11:49 include -> ../../../include/dpdk
  lrwxrwxrwx 1 root root   12 Dec 11 11:49 lib -> ../../../lib     # WRONG LINK

This happens because the directory is not created. Moreover, it makes
sense for install-sdk to install the libs, which could be necessary to
build something against dpdk. After the patch, the link is correct and
the *.a libs are properly installed:

  # ls -l /tmp/toto/usr/local/share/dpdk/x86_64-native-linuxapp-gcc
  drwxr-xr-x 2 root root 4096 Dec 11 11:53 app
  lrwxrwxrwx 1 root root   21 Dec 11 11:53 include -> ../../../include/dpdk
  lrwxrwxrwx 1 root root   12 Dec 11 11:53 lib -> ../../../lib
  # ls -l /tmp/toto/usr/local/share/dpdk/x86_64-native-linuxapp-gcc/lib/
  -rw-r--r-- 1 root root   1378 Dec  8 14:08 libdpdk.a
  -rw-r--r-- 1 root root 123380 Dec  8 14:02 librte_acl.a
  -rw-r--r-- 1 root root   3406 Dec  8 14:02 librte_bitratestats.a
  [...]

Fixes: 6efca1e9f873 ("mk: split install rule")
Cc: stable@dpdk.org
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>