FR: Allow for multi-line `cmdline` params with backslashes.

It would be really nice if you could have a fairly readable config file. From this mess:

```
cmdline: root=UUID=bcb5ce41-9013-49e6-940d-8d5bc8c586fd rw rootfstype=f2fs loglevel=11 splash zswap.enabled=0
```

to:

```
cmdline: root=UUID=bcb5ce41-9013-49e6-940d-8d5bc8c586fd rw rootfstype=f2fs loglevel=11 splash \
    zswap.enabled=0
```