Fix minor build error related to detecting sha1sum
Fix minor build error related to detecting sha1sum

$(error .. ) expression evaluates to empty string which means it
cannot be preceeded with TAB character as it is treated as command then.

See http://stackoverflow.com/a/4713737

This obsoleted #1838

gambol99/kops,chrislovecnm/kops,sethpollack/kops,pdh/kops,BradErz/kops,blakebarnett/kops,MadDogTechnology/kops,ikropotov/kops,zytek/kops,WillemMali/kops,tsupertramp/kops,pdh/kops,WillemMali/kops,justinsb/kops,leandrocr/kops,faraazkhan/kops,WillemMali/kops,tsupertramp/kops,DualSpark/kops,tsupertramp/kops,luomiao/kops,ikropotov/kops,ikropotov/kops,gambol99/kops,luomiao/kops,chrislovecnm/kops,BradErz/kops,kris-nova/kops,mad01/kops,sp-borja-juncosa/kops,faraazkhan/kops,leandrocr/kops,DualSpark/kops,sethpollack/kops,zytek/kops,DualSpark/kops,aledbf/kops,reactiveops/kops,luomiao/kops,dacohen/kops,ikropotov/kops,sp-borja-juncosa/kops,faraazkhan/kops,andrewsykim/kops,sp-borja-juncosa/kops,aledbf/kops,reactiveops/kops,luomiao/kops,justinsb/kops,aledbf/kops,kris-nova/kops,blakebarnett/kops,chrislovecnm/kops,zytek/kops,faraazkhan/kops,blakebarnett/kops,mad01/kops,WillemMali/kops,dacohen/kops,austinmoore-/kops,blakebarnett/kops,MadDogTechnology/kops,mad01/kops,kubernetes/kops,leandrocr/kops,robinpercy/kops,justinsb/kops,BradErz/kops,BradErz/kops,dacohen/kops,zytek/kops,andrewsykim/kops,tsupertramp/kops,gambol99/kops,robinpercy/kops,robinpercy/kops,austinmoore-/kops,andrewsykim/kops,kubernetes/kops,andrewsykim/kops,dacohen/kops,sethpollack/kops,robinpercy/kops,sethpollack/kops,DualSpark/kops,MadDogTechnology/kops,austinmoore-/kops,kris-nova/kops,gambol99/kops,pdh/kops,sp-borja-juncosa/kops,austinmoore-/kops,reactiveops/kops,kubernetes/kops,reactiveops/kops,leandrocr/kops,kris-nova/kops,pdh/kops,chrislovecnm/kops,MadDogTechnology/kops,aledbf/kops,mad01/kops