mk: fix static build on Fedora 22

On Fedora 22, the "ar" binary operates by default in deterministic mode,
making the "u" parameter irrelevant, and leading to warning messages
getting printed in the build output like below.

  INSTALL-LIB librte_kvargs.a
ar: `u' modifier ignored since `D' is the default (see `U')

There are two options to remove these warnings:
* add in the "U" flag to make "ar" non-deterministic again
* remove the "u" flag to have all objects always updated

This patch takes the second approach. It also explicitly adds in the "D"
flag to make behaviour consistent across different distributions which
may have different defaults.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>