Update_server should provide current hashes of flash banks.

[lzrd]

RoT update_server should provide its current view of the stage0, A, and B flash banks.

On RoT, the unused trailing flash sectors are undefined to allow for flexibility in their future use.
The trailing sectors may be in the erased state or may hold content.

There is also a difference between the canonical byte stream that is signed and the byte stream of the contents of a flash bank which would include the signature block and may include trailing bytes in the last sector as well.

The NXP LPC55Sx also skips over one header field (IIRC) when computing the hash for signature verification.

Since we care about the contents of unused flash sectors because of their potential use in exfiltration attacks or for storing malicious code or data, we want the entire bank hashed.

This creates a bit more work during update to be able to predict the layout in the target device and hash given that we already have code to mirror the LPC55 signature verification routine.

It is suggested that update server return something like RotFlashContents below:

struct BankHash {
physaddr: u32,
length: u32,
hash: [u8; 32], // Make it clear what algorithm is in use with an appropriate type.
}

struct RotFlashContents {
stage0: BankHash,
a: BankHash,
b: BankHash,
}

stage1 (a.k.a. pre-kernel, a.k.a.) already computes the bank hashes prior to update_server's start. Update_server's primary function is to mutate those contents which makes the original stage1 information inaccurate during an update procedure.

Looking ahead, we would also want SPFlashContents, but that speculates on having the RoT implement measurement and bank selection policy enforcement against the SP using SWD. That's an issue for another day.