mk: add missing scripts directory in install directory

mk: add missing scripts directory in install directory

Trying to install headers for an external library using DPDK exported makefile
rte.extshared.mk results in following error :

$ cd dpdk
$ make install DESTDIR=/home/marchand/myapp/staging/plop T=x86_64-default-linuxapp-gcc
$ cd ~/myapp
$ make RTE_SDK=/home/marchand/myapp/staging/plop RTE_TARGET=x86_64-default-linuxapp-gcc
  CC plop.o
  LD plop.so
  SYMLINK-FILE include/plop.h
/bin/sh:
/home/marchand/myapp/staging/plop/scripts/relpath.sh: No such file or directory
ln: `/home/marchand/myapp/build/include' and `./include' are the same file
make[1]: *** [/home/marchand/myapp/build/include/plop.h] Error 1
make: *** [all] Error 2

This comes from the fact that DPDK only installs its mk/ directory while some
makefiles require the scripts/ directory content as well.

So install missing files from scripts/.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>