The pins P5.2, P5.1, P5.3 configured as SPI pins are only routed to
USART1 and not to USART0, but previously USART0 was configured as
peripheral backing the bus. This fixes the peripheral configuration by
changing it to USART1.
This is quite unfortunate as USART1 is also used to provide the UART
interface used for `stdio`. Hence, one can either use `stdio` or SPI.
A feature conflict between UART and SPI has therefore been added.
Note that while it would be possible to use P3.2, P3.1, P3.3 to provide
SPI with USART0, this would not work in practise: P3.1 and P3.3 are
connected to the CC1020 transceiver.
Switching to P3.4/P3.5 for UART to provide it using USART 0 would also
resolve the resource conflict. However, these pins are not available
via any of the header and would require soldering the UART<->USB
adapter directly to the pins of the MCU chip on the PCB. It is therefore
much more user friendly to keep the UART bus backed by USART1 to use
pins P3.6 and P3.7 that are easily accessible via the pin header.
