build: allow packagers to disable stage1 build
build: allow packagers to disable stage1 build

RKT_STAGE1_USR_FROM=none ./build

When rkt is built in this way, users will need to get stage1.aci from
another source and either move it to the correct directory or use the
option "-stage1-image=".

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