mk: in install rule, don't overwrite .config if it already exists
mk: in install rule, don't overwrite .config if it already exists

This allows the user to prepare a configuration with make config
before using make install.

Signed-off-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>
Acked-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

john-mcnamara-intel/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,msune/dpdk,mixja/dpdk,msune/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,venkynv/dpdk-mirror,venkynv/dpdk-mirror,tsphillips/dpdk-fork,tsphillips/dpdk-fork,mixja/dpdk,mixja/dpdk,msune/dpdk