This decreases the retrieval time for a frame of 102 bytes from
660 us to 384 us, corresponding to a speed change from about
1.26 Mbps to 2.17 Mbps (102 bytes plus 2 bytes overhead).
- spi.h (spi_off), spi.c: disable the UART
- spi.c (spi_initialized, spi_begin, spi_init): initialize the UART
if necessary
- board_app.c (gpio): call spi_off to disable the UART instead of
open-coding the operation
- board_app.c (gpio): don't explicitly re-enable the UART but defer it
to the next communication
