Toyota ACC_CONTROL PERMIT_BRAKING and ACCEL_CMT_ALT speculated definition

[nelsonjchen]

Not sure if these names are accurate. Also not quite sure of their effect other than existing openpilot's usage (or non-usage). These are the best names I can guess to give them for now. They're better than SET_ME_1 and/or nothing at least?

So, might as well give these signals a name, even if it might not be right, and make some comments on them. Hopefully it makes looking at these in Cabana a little less mysterious.

Unfortunately, merging this in will necessitate some changes to openpilot for tests and the toyota CAN for the existing SET_ME_1 signal in the accel command: https://github.com/commaai/openpilot/blob/ad2c54e9916301d63907d96c3e9dec046bd016b1/selfdrive/car/toyota/toyotacan.py#L37 . ACCEL_CMD_ALT can be left unimplemented as a value of 0 for it is accepted fine like current openpilot.

Drive from an openpilot branch that copies ACCEL_CMD to ACCEL_CMD_ALT and sets PERMIT_BRAKING to high when there's a lead car like stock ACC on a 2020 Corolla: https://my.comma.ai/cabana/?route=1ce1b041fb068985%7C2020-05-03--12-28-37&exp=1620090014&sig=subo0Dfr9iy9%2Fyd36VVHGNPXA9v8u%2BlVxPETS2Xyep4%3D&max=16&url=https%3A%2F%2Fchffrprivate-vzn.azureedge.net%2Fchffrprivate3%2Fv2%2F1ce1b041fb068985%2F2353d33549b4b1b7c6898bf665188c06_2020-05-03--12-28-37

The above branch I used is just experimental and that's the only drive recording I have on hand right now for viewing.

[pd0wm]

PERMIT_BRAKING is what the stock system uses to switch between engine braking and friction brakes? If I recall correctly the stock system only uses friction brakes when there is a lead?