build: Invalidate downloaded pxe.img if URL to it changes
build: Invalidate downloaded pxe.img if URL to it changes

Also, remove unnecessary quotes from the variables - double quotes
have no special meaning in make and potentially messes up invalidation
of systemd_version file for example, because v222 != "v222".

jasonchaffee/rkt,sgotti/rkt,rkt/rkt,liugenping/rkt,adfernandes/rkt,jakon89/rkt,cab105/rkt,tmrts/rkt,tmrts/rkt,achanda/rkt,euank/rkt,jasonchaffee/rkt,kinvolk/rkt,shobhit85/rkt,linki/rkt,kinvolk/rkt,dgonyeo/rkt,shobhit85/rkt,jonboulle/rkt,jonboulle/rkt,Shashwatsh/rkt,kinvolk/rkt,shobhit85/rkt,s-urbaniak/rkt,jjlakis/rkt,jasonchaffee/rkt,linki/rkt,joshix/rkt,Shashwatsh/rkt,squeed/rkt,Wilybald/rkt,liugenping/rkt,yifan-gu/rocket,ybubnov/rkt,akshaykarle/rkt,blixtra/rkt,Wilybald/rkt,jzelinskie/rkt,mike1729/rkt,kinvolk/rkt,Shashwatsh/rkt,akshaykarle/rkt,intelsdi-x/rkt,coreos/rkt,intelsdi-x/rkt,krnowak/rkt,s-urbaniak/rkt,euank/rkt,sinfomicien/rkt,nhlfr/rkt,yifan-gu/rocket,monstermunchkin/rkt,yifan-gu/rkt,jonboulle/rkt,endocode/rkt,linki/rkt,rkt/rkt,sinfomicien/rkt,sinfomicien/rkt,blixtra/rkt,sstabellini/rkt,yifan-gu/rkt,s-urbaniak/rkt,squeed/rkt,markllama/rkt,alban/rkt,sgotti/rkt,dgonyeo/rkt,jzelinskie/rkt,monstermunchkin/rkt,kbrwn/rkt,sgotti/rkt,squeed/rkt,intelsdi-x/rkt,cab105/rkt,euank/rkt,jzelinskie/rkt,joshix/rkt,ybubnov/rkt,adfernandes/rkt,endocode/rkt,il9ue/rkt,ybubnov/rkt,adfernandes/rkt,matthaias/rkt,Wilybald/rkt,il9ue/rkt,matthaias/rkt,coreos/rkt,rkt/rkt,jjlakis/rkt,il9ue/rkt,dgonyeo/rkt,achanda/rkt,mike1729/rkt,Wilybald/rkt,tmrts/rkt,jakon89/rkt,akshaykarle/rkt,nhlfr/rkt,jjlakis/rkt,nhlfr/rkt,sstabellini/rkt,kbrwn/rkt,achanda/rkt,cab105/rkt,lucasallan/rkt,lucasallan/rkt,endocode/rkt,krnowak/rkt,joshix/rkt,blixtra/rkt,alban/rkt,kbrwn/rkt,mike1729/rkt,krnowak/rkt,markllama/rkt,matthaias/rkt,lucasallan/rkt,coreos/rkt,sstabellini/rkt,lucasallan/rkt,liugenping/rkt,markllama/rkt,joshix/rkt,rkt/rkt,monstermunchkin/rkt,jakon89/rkt,yifan-gu/rkt,yifan-gu/rocket,alban/rkt