Introduce concept of read-only variables

[Scottapotamas]

Background

For many uses, the microcontroller may not want to provide write access to a given variable.

This could be values stored in ROM, non-editable values like calibration registers, or values which shouldn't be modified by the UI but managing a copied array is heavier than needed (situations where DMA is writing sensor values into a ring buffer at a high rate, etc).

The eUIMessage object structure currently uses a uint8 for the type metadata, but we only use a few bits of that byte.

Proposal and impact

I propose using one of those spare 'type' bits to specify a writable flag.

• [x] This means there should be no hit to memory or ram in the structure storage.
• [x] Checks can be done with bitwise maths at low computational cost.
• [x] The RO state is enforced at the microcontroller side, rather than 'asking nicely'.
• [x] We can use macros to make this easy for the developer.
• [x] The UI handshake process should distinguish the read-only variables

Affected behaviours

• We don't have a spare bit in the messages to indicate this on a per-packet basis, and we wouldn't want to.
• Its going to be enforced at compile time, so its immutable anyway.
• The handshaking process needs to give the UI an indication of writable values in some way.
• We can probably resolve this by splitting the message transfer process to the micro, where writable and read-only variables are split into two groups.
• The overhead from two sets is one additional message, so the overall bandwidth cost is approximately one message (13 bytes?).

The user interface then needs to do something useful with this information, but can be addressed later and in the correct issues trackers.

[Scottapotamas]

946d8de85a8c3cabd71bd821c0182aa9b166acbc adds support for read only variables.

Remaining work is to modify the handshake process to provide this information to the UI.

[Scottapotamas]

Refactored the handshake callbacks as part of the (off-branch) 2ad87e1e5f18b49604e490035124f50908a3a602 commit. This code has not been tested on hardware, and doesn't have written unit tests, but does compile without warnings.

Breaking changes:

• The previous message ID's used ("dm" and "dv") have been replaced by "dmr"/"dmw" and "dvr"/"dvw" for the read-only and the read/writeable ones.
• It's up to the UI to request each set and handle the responses accordingly.
• The previous "dms" for indicating start of the stream (do we actually need that), is now "dmrs"/"dmws".
• The previous packets containing the list of messageID's "dml", is now namespaced "dmrl"/"dmwl".

Remaining work:

• [x] Test on hardware,
• [x] Test against the UI once support is available in the connections manager,
• [x] Write unit tests for each of the callbacks and expected behaviour once stabilised,
• [x] Merge a working implementation into master.

[Scottapotamas]

This was merged in #62, and #47 somewhat encompases the higher level test requirements.

UI support should be tracked outside the scope of this repo.