build: Use per-flavor variables in usr_from_*
build: Use per-flavor variables in usr_from_*

It is possible to do, because usr_from_* does not use any global
variables (STAGE1_USR_STAMPS, ACIROOTFSDIR, etc...) anymore.

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