Update stage0

[lzrd]

Support update of RoT bootloader.

Includes always calculating FWID and signature check at RoT boot time.
Messages for SP and RoT Hubris updates are backward compatible.

See https://github.com/oxidecomputer/sprot-e2e for test scripts that exercise update and rollback of Hubris and Bootleby on SP and RoT.

A new RoTBootInfo mesage is plumbed that includes info on the four RoT flash banks.

The ImageError enum gives detailed information on invalid RoT flash bank contents.

This is a draft PR because it needs to be coordinated with hubris and omicron changes so as not to break people's development environments.

Closes #208

[mkeeter]

Do we ever care about the in-memory representation of this enum? If not, I'd recommend removing the = 1, because it suggests that we care about the in-memory representation.

Note that the = 1 does not change how this object will be serialized. For example:

#[derive(serde::Serialize)]
enum Foo {
    One = 1,
    Two,
}

fn main() {
    let b = bincode::serialize(&Foo::One).unwrap();
    println!("{b:?}") // prints [0, 0, 0, 0]
}

[lzrd]

You're right, we don't care about the in-memory representation.