stage1: exit code: use systemctl exit on coreos flavor
stage1: exit code: use systemctl exit on coreos flavor

The exit status can be propagated from apps by using "systemctl exit"
since systemd v227. rkt checks the systemd version to know whether it
can use "systemctl exit". However, systemd from coreos is back porting
the patches, so we skip the systemd version check on the coreos flavor.

See https://github.com/coreos/rkt/issues/1460

akshaykarle/rkt,cab105/rkt,kinvolk/rkt,jonboulle/rkt,achanda/rkt,intelsdi-x/rkt,tmrts/rkt,akshaykarle/rkt,achanda/rkt,monstermunchkin/rkt,intelsdi-x/rkt,rkt/rkt,lucasallan/rkt,s-urbaniak/rkt,sgotti/rkt,monstermunchkin/rkt,intelsdi-x/rkt,kinvolk/rkt,s-urbaniak/rkt,alban/rkt,sgotti/rkt,joshix/rkt,Wilybald/rkt,rkt/rkt,euank/rkt,markllama/rkt,markllama/rkt,sstabellini/rkt,jjlakis/rkt,lucasallan/rkt,mike1729/rkt,Wilybald/rkt,kinvolk/rkt,jakon89/rkt,coreos/rkt,sstabellini/rkt,mike1729/rkt,s-urbaniak/rkt,Wilybald/rkt,euank/rkt,coreos/rkt,jakon89/rkt,jonboulle/rkt,achanda/rkt,kinvolk/rkt,shobhit85/rkt,akshaykarle/rkt,shobhit85/rkt,squeed/rkt,tmrts/rkt,nhlfr/rkt,jjlakis/rkt,lucasallan/rkt,joshix/rkt,jjlakis/rkt,lucasallan/rkt,yifan-gu/rkt,yifan-gu/rkt,cab105/rkt,joshix/rkt,alban/rkt,yifan-gu/rkt,coreos/rkt,Wilybald/rkt,rkt/rkt,matthaias/rkt,ybubnov/rkt,Shashwatsh/rkt,Shashwatsh/rkt,ybubnov/rkt,tmrts/rkt,squeed/rkt,nhlfr/rkt,sgotti/rkt,matthaias/rkt,shobhit85/rkt,squeed/rkt,yifan-gu/rocket,dgonyeo/rkt,yifan-gu/rocket,euank/rkt,jakon89/rkt,cab105/rkt,alban/rkt,Shashwatsh/rkt,yifan-gu/rocket,sstabellini/rkt,jonboulle/rkt,matthaias/rkt,monstermunchkin/rkt,dgonyeo/rkt,joshix/rkt,ybubnov/rkt,markllama/rkt,mike1729/rkt,rkt/rkt,dgonyeo/rkt,nhlfr/rkt