sled-agent could use optional dependency on baseline service

I noticed 13 messages in my sled-agent log leading up to a successful Nexus handoff (i.e., the success case):

23:16:20.047Z WARN SledAgent (ServiceManager): Failed to initialize switch zone: Failed to install zone 'oxz_switch' from '/opt/oxide/switch.tar.gz': Failed to execute zoneadm command 'Install' for zone 'oxz_switch': Failed to parse command output: exit code 1
    stdout:
    INFO: omicron: installing zone oxz_switch @ "/zone/oxz_switch"...
    INFO: omicron: replicating /usr tree...
    INFO: omicron: replicated /usr: 23247 files, 372443399, bytes, 2025 msec
    INFO: omicron: replicating /lib tree...
    INFO: omicron: replicated /lib: 256 files, 3273194, bytes, 28 msec
    INFO: omicron: replicating /sbin tree...
    INFO: omicron: replicated /sbin: 0 files, 0, bytes, 2 msec
    INFO: omicron: pruning SMF manifests...
    INFO: omicron: pruning global-only files...
    stderr:
    Error: could not locate "gzonly.txt" in any baseline directory
    file = sled-agent/src/services.rs:4362

It looks like those messages predate the time when the omicron/baseline service came online:

root@g0:~# svcs baseline
STATE          STIME    FMRI
online         23:16:20 svc:/system/omicron/baseline:default

After that timestamp, the messages stopped and setup proceeded.

@jclulow writes:

The svc:/system/omicron/baseline:default is essentially an enabling workaround for running Omicron bits on mutable install-to-disk systems like workstations, etc. The service doesn't exist at all on production systems because, as described in omicron1(7), we create the baseline during the build of the ramdisk image and ship it in place.

With this in mind, I think you could give sled-agent an optional_all dependency on the baseline service, which should give the correct startup sequencing on a development system but be silently ignored in the production ramdisk.

Originally posted by @davepacheco in https://github.com/oxidecomputer/omicron/issues/5702#issuecomment-2287401871

Start by locating sled-agent's service definition and inspecting how its existing SMF service dependencies are declared. Confirm the optional baseline dependency is used for development startup sequencing while remaining absent or harmless on production ramdisk systems. Done means the relevant startup warnings no longer occur when baseline comes online later.