configure_bootloaders: only add a single serial console boot arg
configure_bootloaders: only add a single serial console boot arg

Only one console of each type (virtual, serial, etc.) is supported
in the boot args. Only add ttyS0 if no other serial consoles have
been provided.

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