This 2-in-1 PR started with the goal of support the Bangle.js 2
smartwatch with *no USB*.
* Adds "secure" DFU build support with a committed private key.
* Adds 3-bit color support with one dummy bit for the JDI memory display
* Allows nrf boards to have a board_background_task() run in RUN_BACKGROUND_TASK.
This is needed because the Bangle.js 2 uses the watchdog to reset.
* Renamed port_background_task() to port_background_tick() to indicate it
runs on tick, not RUN_BACKGROUND_TASK.
* Marks serial connected when the display terminal is inited. This means
that safe mode messages show up on the display.
ACep, 7-color epaper displays also pack 3 bits in 4. So, I added that
support as well.
* Adds 3-bit ACeP color support for 7-color e-paper displays. (Not
watch related but similar due to color depth.)
* Allows a refresh sequence instead of a single int command. The 7" ACeP
display requires a data byte for refresh.
* Adds optional delay after resetting the display. The ACeP displays
need this. (Probably to load LUTs from flash.)
* Adds a cleaning phase for ACeP displays before the real refresh.
For both:
* Add dither support to Palette.
* Palette no longer converts colors when set. Instead, it caches
converted colors at each index.
* ColorConverter now caches the last converted color. It should make
conversions faster for repeated colors (not dithering.)
This targets the 64-bit CPU Raspberry Pis. The BCM2711 on the Pi 4
and the BCM2837 on the Pi 3 and Zero 2W. There are 64-bit fixes
outside of the ports directory for it.
There are a couple other cleanups that were incidental:
* Use const mcu_pin_obj_t instead of omitting the const. The structs
themselves are const because they are in ROM.
* Use PTR <-> OBJ conversions in more places. They were found when
mp_obj_t was set to an integer type rather than pointer.
* Optimize submodule checkout because the Pi submodules are heavy
and unnecessary for the vast majority of builds.
Fixes#4314
