This uses the crystal to clock the RTC on boards which have a crystal.
Disable clock generator 2 which was enabled in commit
8e2080411f ("atmel-samd: Add rtc module support").
samd51 differs from samd21 when it comes to the RTC clock. samd51 doesn't
have an explicit clock peripheral so no need for a clock generator.
The same commit didn't even setup XOSC32K correctly, it missed EN1K and XTALEN.
The RTC uses the 1k clock output, so enable it on the OSCULP32K even if it works without it.
1. UART: ported to ASF4. Allow rx-only and tx-only. Add .baudrate r/w property.
2. Make NeoPixel timing deterministic by turning off caches during NeoPixel writes.
3. Incorporate asf4 updates:
a. async USART driver
b. bringing Atmel START configuration closer to what we use
c. Clock initialization order now specified by CIRCUITPY_GCLK_INIT_1ST and _LAST.
4. supervisor/port.c: Move commented-out clock-test pin setting to correct location.
This introduces a new spi_flash_api.h that works for both SPI and
QSPI. The previous spi_flash functions are now called external_flash
to minimize confusion.
Also, fixed pin mappings for rev B Metro M4:
swap PA12 and PA13 on SPI 2x3 header
swap A3 and A5
Comment out all frozen modules in CPX again to make room while waiting
for SPI flash.
* Introduce a python script to generate the USB descriptor instead of
a bunch of C macros. In the future, we can use this dynamically in
CircuitPython.
* Add support for detecting read-only mass storage mounts.
Fixes#377
