config: fix validation of drop-in names
config: fix validation of drop-in names

dgonyeo/ignition,trnubo/ignition,AlexNPavel/ignition,jonboulle/ignition,higebu/ignition,jonboulle/ignition,AlexNPavel/ignition,jonboulle/ignition,coreos/ignition,coreos/ignition,mischief/ignition,dgonyeo/ignition,dgonyeo/ignition,AlexNPavel/ignition,mischief/ignition,marineam/ignition,trnubo/ignition,trnubo/ignition,higebu/ignition,higebu/ignition,coreos/ignition,marineam/ignition,ajeddeloh/ignition,mischief/ignition,ajeddeloh/ignition,ajeddeloh/ignition,marineam/ignition