stages/disks: Wait for uevents triggered by mkfs to settle
stages/disks: Wait for uevents triggered by mkfs to settle

The disks stage could complete with udev events still in flight. When
running mkfs.btrfs to overwrite an existing filesystem, this could cause
by-label symlinks to be temporarily deleted while other systemd units
were expecting them to exist.

AlexNPavel/ignition,dgonyeo/ignition,AlexNPavel/ignition,dgonyeo/ignition,coreos/ignition,coreos/ignition,AlexNPavel/ignition,ajeddeloh/ignition,ajeddeloh/ignition,dgonyeo/ignition,coreos/ignition,ajeddeloh/ignition