build: change default driver installation directory

build: change default driver installation directory

As part of the effort of consolidating the DPDK installation bits and
pieces across distros, set the default directory of lib/ where PMDs get
installed to dpdk/pmds-XX.YY. It's necessary to have a versioned
subdirectory as multiple ABI revisions might be installed at the same
time, so having a fixed name will cause trouble with the autoload
feature.
Small refactor with parsing and saving the major version to a variable,
since it's now used in 3 different places.

Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Timothy Redaelli <tredaelli@redhat.com>