[Unit]
Description=Wait for root filesystem to be read-write
DefaultDependencies=no
Before=sysinit.target
After=systemd-fsck-root.service

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/bash -c 'for i in $(seq 1 30); do touch /.rw_test 2>/dev/null && rm -f /.rw_test && exit 0; sleep 2; done; exit 1'

[Install]
WantedBy=sysinit.target