build: Introduce per-flavor variables
build: Introduce per-flavor variables

Nothing uses it right now, but following commits will change uses of
global STAGE1_ variables to them.

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