After this change, the following program works for me on the MIMXRT1010-EVK:
```python
import pwmio
import board
p = pwmio.PWMOut(board.D13, frequency=1_000_000, variable_frequency=True)
p.duty_cycle = 32868
while True:
pass
```
Querying and varying the duty_cycle and frequency work as well.
The lowest frequency obtainable is about 2kHz; there is an additional
divider which would allow lower PWM frequencies (I think 1kHz is important
for servos?)
Something odd happens with very low duty cycles, such as
```python
>>> p.frequency = 2000
>>> p.duty_cycle = 2
```
instead of a symmetrical waveform, it's asymmetrical. With `duty_cycle=4`,
the effect disappears. The reason for this is probably hidden in the
datasheet, but could affect servos or other things that count pulse
widths.
This allows more options to be overridden to 0 in an mpconfigboard.mk.
Improved:
* FRAMEBUFFERIO, FULL_BUILD, BITOPS, PWMIO, RGBMATRIX, OTARYIO, PULSEIO, WATCHDOG
Still problematic (pull requests welcome):
* RP2PIO & NEOPIXEL_WRITE, possibly only if a status neopixel is defined
* BITBANGIO, possibly only if BUSIO is enabled
* RTC
Incidentally, with RP2PIO & NEOPIXEL_WRITE, BITBANGIO, and RTC re-enabled I get
```
323956 bytes used, 720524 bytes free in flash firmware space out of 1044480 bytes (1020.0kB).
12072 bytes used, 250072 bytes free in ram for stack and heap out of 262144 bytes (256.0kB).
```
Closes#4515
Set the betweenTransferDelay to the SCK low-time, to avoid long pauses
between bytes (transfers) while preventing the last SCK cycle in a byte
from being a runt pulse.
Compared to an earlier revision of this change, which just set the delays
all to zero, this doesn't break using an AirLift, which was sensitive
to the runt pulses (the simple loopback-wire test didn't detect the problem)
without this, the baud rate could be wrong; in my testing, it was
low by a factor of 2 when requesating baudrate=1_000_000 (1MHz).
When passing the baudrate in to LPSPI_MasterInit, the setting is made
automatically, but LPSPI_MAster_SetBaudRate just returns it via the
out-parameter tcrPrescaleValue.
