mkosi: in UsrOnly=yes mode, generate a mount.usr= kernel cmdline switch

mkosi: in UsrOnly=yes mode, generate a mount.usr= kernel cmdline switch

So far, we relied on gpt-auto's logic to automatically find the root fs
on the image. This is not supported for finding the /usr partition
though, hence generate a mount.usr= kernel cmdline option to address
this.

(And it's unlikely we'll add auto-discovery for /usr, since we cannot
realistically decide whether its worth waiting for /usr. After all we
*know* we can't proceed without a rootfs, hence it makes sense to
generically wait for one if we never got one configured one. But a /usr
partition is an optional thing, hence it doesn't make sense to wait for
it if we didn#t get told about it)