build: Handle systemd git repo and version in single place
build: Handle systemd git repo and version in single place

Maybe also a bit more readable.

jonboulle/rkt,sinfomicien/rkt,alban/rkt,alexhersh/rkt,mjg59/rkt,sgotti/rkt,nhlfr/rkt,rkt/rkt,jasonchaffee/rkt,kinvolk/rkt,marineam/rkt,kbrwn/rkt,nhlfr/rkt,monstermunchkin/rkt,dgonyeo/rkt,euank/rkt,adfernandes/rkt,alexhersh/rkt,tmrts/rkt,achanda/rkt,jjlakis/rkt,ybubnov/rkt,markllama/rkt,cab105/rkt,joshix/rkt,kbrwn/rkt,jonboulle/rkt,tv42/rkt,jasonchaffee/rkt,lucasallan/rkt,jakon89/rkt,mike1729/rkt,sgotti/rkt,liubq919/rkt,smarvin/rkt,lee212/rkt,matthaias/rkt,jjlakis/rkt,ybubnov/rkt,chrisseto/rkt,liugenping/rkt,markllama/rkt,shobhit85/rkt,joshix/rkt,dalanlan/rkt,mike1729/rkt,alban/rkt,squeed/rkt,iankronquist/rkt,chrisseto/rkt,tmrts/rkt,jasonchaffee/rkt,s-urbaniak/rkt,adfernandes/rkt,sgotti/rkt,jjlakis/rkt,iaguis/rkt,sstabellini/rkt,dgonyeo/rkt,dalanlan/rkt,Wilybald/rkt,intelsdi-x/rkt,linki/rkt,intelsdi-x/rkt,luebken/rkt,tmrts/rkt,sinfomicien/rkt,intfrr/rkt,akshaykarle/rkt,jzelinskie/rkt,akshaykarle/rkt,iaguis/rkt,yifan-gu/rocket,intfrr/rkt,kinvolk/rkt,adfernandes/rkt,dalanlan/rkt,blixtra/rkt,blixtra/rkt,coreos/rkt,jzelinskie/rkt,yifan-gu/rocket,intfrr/rkt,Shashwatsh/rkt,lucasallan/rkt,cab105/rkt,joshix/rkt,yifan-gu/rkt,squeed/rkt,squeed/rkt,sstabellini/rkt,dgonyeo/rkt,rkt/rkt,blixtra/rkt,krnowak/rkt,matthaias/rkt,liubq919/rkt,alexhersh/rkt,krnowak/rkt,euank/rkt,liubq919/rkt,Shashwatsh/rkt,Shashwatsh/rkt,smarvin/rkt,matthaias/rkt,smarvin/rkt,ybubnov/rkt,monstermunchkin/rkt,sstabellini/rkt,markllama/rkt,marineam/rkt,monstermunchkin/rkt,tv42/rkt,cab105/rkt,euank/rkt,mjg59/rkt,jakon89/rkt,yifan-gu/rkt,kinvolk/rkt,lee212/rkt,s-urbaniak/rkt,luebken/rkt,coreos/rkt,sinfomicien/rkt,Wilybald/rkt,tv42/rkt,marineam/rkt,endocode/rkt,kbrwn/rkt,linki/rkt,Wilybald/rkt,lee212/rkt,endocode/rkt,lucasallan/rkt,lucasallan/rkt,shobhit85/rkt,yifan-gu/rocket,intelsdi-x/rkt,iankronquist/rkt,jzelinskie/rkt,Yhgenomics/rkt,coreos/rkt,shobhit85/rkt,rkt/rkt,iankronquist/rkt,s-urbaniak/rkt,Yhgenomics/rkt,endocode/rkt,krnowak/rkt,achanda/rkt,chrisseto/rkt,jonboulle/rkt,alban/rkt,kinvolk/rkt,liugenping/rkt,rkt/rkt,iaguis/rkt,luebken/rkt,akshaykarle/rkt,il9ue/rkt,linki/rkt,mike1729/rkt,jakon89/rkt,joshix/rkt,Yhgenomics/rkt,il9ue/rkt,liugenping/rkt,Wilybald/rkt,achanda/rkt,mjg59/rkt,il9ue/rkt,nhlfr/rkt,yifan-gu/rkt