packrat: Read caboose metadata into ereports

[hawkw]

Follow-up from #2343

PR #2343 removes the hubris_archive_id field from ereport metadata, as we have determined that this ought not be used to identify Hubris except in the case of firmware updates. If this is being removed, though, we really ought to have other metadata fields identifying the Hubris image. Therefore, this commit adds fields from the caboose (in particular, the BORD, VERS, and GITC tags) to the ereport metadata message. These fields are read from the caboose every time metadata is refreshed, in order to avoid buffering them in packrat, which would duplicate data already in flash and didn't seem necessary as metadata refreshes occur infrequently (on SP reset/MGS restart).

All of these fields are optional, and if any of them are not present or could not be read successfully, we send a CBOR null. Additionally, I've nested all of them under a hubris_caboose field, which is null if the image has no caboose whatsoever. This way, we can differentiate between images with no caboose and images where none of the tags we read into the metadata message could be found. I'm open to being convinced this is unnecessary, but it seemed worthwhile, and since the metadata message doesn't compete for space in the ereport ringbuffer, we can be a bit more verbose here, provided it fits in a UDP datagram.

Naturally, every app.toml where Packrat produces ereports needed to be updated to allow packrat to read from the caboose. Packrat also uses a bit more stack in order to do this.

For example, here's output from a Gimletlet with caboose fields (including a fake version tag) in its ereport metadata:

eliza@hekate ~/Code/oxide/hubris $ faux-mgs --interface eno1np0 --discovery-addr '[fe80::0c1d:deff:fef0:d922]:11111' ereports
Jan 06 10:19:21.564 INFO creating SP handle on interface eno1np0, component: faux-mgs
Jan 06 10:19:21.565 INFO initial discovery complete, addr: [fe80::c1d:deff:fef0:d922%2]:11111, interface: eno1np0, socket: control-plane-agent, component: faux-mgs
restart ID: aecfcbd7-4637-8a9a-ed0b-0d5b60a884e8
restart IDs did not match (requested 00000000-0000-0000-0000-000000000000)
count: 1

ereports:
0x1: {
    "baseboard_part_number": String("LOLNO000000"),
    "baseboard_rev": Number(42),
    "baseboard_serial_number": String("69426661337"),
    "ereport_message_version": Number(0),
    "hubris_caboose": Object {
        "board": String("gimletlet-2"),
        "commit": String("51dac3ec71877d330981cd5167a4aef5fb48311c-dirty"),
        "version": String("42.69.420-eliza-test"),
    },
    "hubris_task_gen": Number(0),
    "hubris_task_name": String("packrat"),
    "hubris_uptime_ms": Number(0),
    "lost": Null,
}