The following error occurs when building with gcc 5.4.1 (debian stretch):
common-hal/busio/UART.c:104:83: error: 'sercom_index' may be used uninitialized in this function [-Werror=maybe-uninitialized]
sercom_insts[rx->sercom[j].index]->USART.CTRLA.bit.ENABLE == 0) ||
It may be related to the addition of rx-only UARTs; gcc is unable
to infer the intended relationship between have_tx and sercom_index
being set (I am still not entirely confident of it myself)
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.
