Introduce `bootloader`/manifest versioning scheme

[vk2seb]

Sometimes there are bootloader changes that are not backwards-compatible with old bitstreams. The manifest and bootloader should agree with each other that they are compatible, before the bootloader tries to boot them.

[vk2seb]

This is kind of already covered by MANIFEST_MAGIC here:

tiliqua/gateware/src/rs/manifest/src/lib.rs

Line 21 in 9efaf3d

pub const MANIFEST_MAGIC: u32 = 0xFEEDBEEF;

We should bump this after the first official release if there is a breaking manifest change.