setup_board: fix compiling toolchains from scratch
setup_board: fix compiling toolchains from scratch

Commit 0d29e735 broke this by not properly initializing the toolchain
ebuild flags when binary packages were disabled.

trnubo/scripts,sigma/coreos-scripts,vmware/coreos-scripts,mjg59/scripts,mischief/scripts,trnubo/scripts,coreos/scripts,sigma/coreos-scripts,zhang0137/scripts,vmware/coreos-scripts,zhang0137/scripts,BugRoger/scripts,nicescale/scripts,marineam/coreos-scripts,mjg59/scripts,mischief/scripts,glevand/coreos--scripts,andrejro/coreos--scripts,endocode/scripts,zhang0137/scripts,BugRoger/scripts,endocode/scripts,gtank/scripts,smilart/scripts,fivethreeo/scripts,glevand/coreos--scripts,andrejro/coreos--scripts,sigma/coreos-scripts,coreos/scripts,marineam/coreos-scripts,nicescale/scripts,gtank/scripts,fivethreeo/scripts,nicescale/scripts,endocode/scripts,fivethreeo/scripts,trnubo/scripts,mischief/scripts,vmware/coreos-scripts,BugRoger/scripts,mjg59/scripts,marineam/coreos-scripts,smilart/scripts,andrejro/coreos--scripts,gtank/scripts,smilart/scripts