build: warn the user if dpdk submodule is missing
build: warn the user if dpdk submodule is missing

If the user tried to use "--enable-dpdk", verify that the dpdk/
submodule was actually retrieved. If not, warn the user and tell
him exactly what he needs to do (namely, "git submodule update --init").

This is easier to understand than an obscure error message about a
missing Makefile...

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@cloudius-systems.com>

xtao/seastar,avikivity/seastar,flashbuckets/seastar,dwdm/seastar,kjniemi/seastar,koolhazz/seastar,koolhazz/seastar,chunshengster/seastar,syuu1228/seastar,shyamalschandra/seastar,avikivity/seastar,mixja/seastar,joerg84/seastar,chunshengster/seastar,mixja/seastar,leejir/seastar,hongliangzhao/seastar,linearregression/seastar,cloudius-systems/seastar,syuu1228/seastar,mixja/seastar,chunshengster/seastar,acbellini/seastar,scylladb/scylla-seastar,leejir/seastar,dwdm/seastar,jonathanleang/seastar,stamhe/seastar,norcimo5/seastar,anzihenry/seastar,joerg84/seastar,printedheart/seastar,bowlofstew/seastar,linearregression/seastar,flashbuckets/seastar,scylladb/scylla-seastar,tempbottle/seastar,shyamalschandra/seastar,scylladb/scylla-seastar,bowlofstew/seastar,printedheart/seastar,linearregression/seastar,scylladb/seastar,flashbuckets/seastar,wildinto/seastar,jonathanleang/seastar,hongliangzhao/seastar,stamhe/seastar,tempbottle/seastar,anzihenry/seastar,cloudius-systems/seastar,kjniemi/seastar,joerg84/seastar,wildinto/seastar,shyamalschandra/seastar,leejir/seastar,scylladb/seastar,sjperkins/seastar,avikivity/seastar,norcimo5/seastar,scylladb/seastar,sjperkins/seastar,kjniemi/seastar,hongliangzhao/seastar,anzihenry/seastar,sjperkins/seastar,syuu1228/seastar,acbellini/seastar,stamhe/seastar,xtao/seastar,tempbottle/seastar,xtao/seastar,dwdm/seastar,dreamsxin/seastar,jonathanleang/seastar,printedheart/seastar,koolhazz/seastar,wildinto/seastar,dreamsxin/seastar,cloudius-systems/seastar,acbellini/seastar,dreamsxin/seastar,norcimo5/seastar,bowlofstew/seastar