.. however, the number of endpoints is only set for SAMD (8).
Other ports need to set the value. Otherwise, the build will show
the message
```
Unable to check whether maximum number of endpoints is respected
```
The font is missing many characters and the build needs the space.
We can optimize font storage when we get a good font.
The serial output will work as usual.
Before this, a background callback that was on the list when
background_callback_reset was called could have ended up in a state
that made it "un-queueable": its "prev" pointer could have been non-NULL.
A background callback must never outlive its related object. By
collecting the head of the linked list of background tasks, this will
not happen.
One hypothetical case where this could happen is if an MP3Decoder is
deleted while its callback to fill its buffer is scheduled.
CALLBACK_CRITICAL_BEGIN is heavyweight, but we can be confident we do
not have work to do as long as callback_head is NULL.
This gives back performance on nRF.
In time, we should transition interrupt driven background tasks out of the
overall run_background_tasks into distinct background callbacks,
so that the number of checks that occur with each tick is reduced.
This restores the ability to remove CDC and/or MSC, at the price of
giving up the new automatic check that USB_DEVICES is correct.
Since devices have to have CDC and MSC to be "CircuitPython",
this is not a facility that is going to be used by any in-tree drivers.
Since Actions passed on the previous commit, where this computed value
was checked against the specified value (if any), this is no net change,
except that we no longer need to specify it for particular boards or
ports.
Few peripherals are actually tested. However, USB, I2C and GPIO seem to work.
Most pins are silkscreened with the "PX00" style, so the board module
only includes the small number that are screened differently.
The default SPI, I2C, and UART are the ones on the EXT2 header. This is
arbitrary, but the I2C on this connector is shared with the on-board I2C
devices and the PCC header, making it the most versatile.
