stage1: flavor=src: fix git clone directory
stage1: flavor=src: fix git clone directory

When compiling the src flavor, systemd will be cloned from the URL given
in $RKT_STAGE1_SYSTEMD_SRC. The Makefile assumed that
$RKT_STAGE1_SYSTEMD_SRC ends with "systemd" but that might not be the
case. Fix the Makefile.

tixxdz/rkt,intelsdi-x/rkt,Yhgenomics/rkt,Yhgenomics/rkt,il9ue/rkt,jjlakis/rkt,tv42/rkt,mike1729/rkt,squeed/rkt,rkt/rkt,mike1729/rkt,joshix/rkt,yifan-gu/rocket,jjlakis/rkt,squaremo/rkt,nhlfr/rkt,rot26/rkt,endocode/rkt,Shashwatsh/rkt,yifan-gu/rocket,ChengTiesheng/rkt,iaguis/rkt,kinvolk/rkt,intelsdi-x/rkt,sgotti/rkt,fdserr/rkt,dgonyeo/rkt,dgonyeo/rkt,s-urbaniak/rkt,jakon89/rkt,maquanyi/rkt,kbrwn/rkt,iaguis/rkt,tixxdz/rkt,tmrts/rkt,cab105/rkt,matthaias/rkt,euank/rkt,monstermunchkin/rkt,alexhersh/rkt,tixxdz/rkt,linki/rkt,nhlfr/rkt,yifan-gu/rkt,coreos/rkt,euank/rkt,Wilybald/rkt,adfernandes/rkt,kbrwn/rkt,marineam/rkt,krnowak/rkt,akshaykarle/rkt,intfrr/rkt,jzelinskie/rkt,jakon89/rkt,nhlfr/rkt,matthaias/rkt,fdserr/rkt,sstabellini/rkt,crawford/rkt,iankronquist/rkt,MattDees/rkt,liugenping/rkt,joshix/rkt,monstermunchkin/rkt,chrisseto/rkt,mjg59/rkt,jonboulle/rkt,alexhersh/rkt,michelangelo13/rkt,Wilybald/rkt,luebken/rkt,coreos/rkt,maquanyi/rkt,markllama/rkt,joshix/rkt,tv42/rkt,bsmr-go/rkt,shobhit85/rkt,linki/rkt,jasonchaffee/rkt,jakon89/rkt,ybubnov/rkt,dalanlan/rkt,rhvgoyal/rkt,luebken/rkt,blixtra/rkt,Shashwatsh/rkt,fdserr/rkt,liugenping/rkt,jonboulle/rkt,mike1729/rkt,dalanlan/rkt,intfrr/rkt,squaremo/rkt,lee212/rkt,dgonyeo/rkt,Wilybald/rkt,monstermunchkin/rkt,iankronquist/rkt,liubq919/rkt,achanda/rkt,squeed/rkt,jasonchaffee/rkt,liubq919/rkt,intelsdi-x/rkt,wparad/rkt,adfernandes/rkt,jasonchaffee/rkt,lucasallan/rkt,achanda/rkt,endocode/rkt,luebken/rkt,lee212/rkt,squaremo/rkt,dalanlan/rkt,Shashwatsh/rkt,rhvgoyal/rkt,maquanyi/rkt,sstabellini/rkt,alexhersh/rkt,blixtra/rkt,crawford/rkt,yifan-gu/rocket,tmrts/rkt,jjlakis/rkt,kinvolk/rkt,sinfomicien/rkt,il9ue/rkt,MattDees/rkt,alban/rkt,rot26/rkt,ChengTiesheng/rkt,liugenping/rkt,rkt/rkt,cab105/rkt,s-urbaniak/rkt,michelangelo13/rkt,sinfomicien/rkt,krnowak/rkt,kinvolk/rkt,chrisseto/rkt,akshaykarle/rkt,akshaykarle/rkt,jzelinskie/rkt,il9ue/rkt,yifan-gu/rkt,sgotti/rkt,lee212/rkt,cab105/rkt,squeed/rkt,euank/rkt,joshix/rkt,crawford/rkt,tv42/rkt,achanda/rkt,smarvin/rkt,marineam/rkt,kinvolk/rkt,rkt/rkt,ybubnov/rkt,Yhgenomics/rkt,yifan-gu/rkt,kbrwn/rkt,blixtra/rkt,shobhit85/rkt,markllama/rkt,jzelinskie/rkt,sstabellini/rkt,liubq919/rkt,iaguis/rkt,iankronquist/rkt,marineam/rkt,alban/rkt,sgotti/rkt,mjg59/rkt,wparad/rkt,adfernandes/rkt,smarvin/rkt,alban/rkt,rhvgoyal/rkt,s-urbaniak/rkt,markllama/rkt,MattDees/rkt,intfrr/rkt,tmrts/rkt,rkt/rkt,Wilybald/rkt,lucasallan/rkt,krnowak/rkt,sinfomicien/rkt,lucasallan/rkt,jonboulle/rkt,linki/rkt,lucasallan/rkt,wparad/rkt,bsmr-go/rkt,shobhit85/rkt,chrisseto/rkt,ybubnov/rkt,coreos/rkt,ChengTiesheng/rkt,mjg59/rkt,smarvin/rkt,rot26/rkt,endocode/rkt,michelangelo13/rkt,bsmr-go/rkt,matthaias/rkt