spicmds: Rework spi config commands

Rework the spi_config commands so that bus configuration and shutdown
message configuration is done separately from the main "config_spi"
command.  This makes the spi configuration more flexible.  It's now
possible to use software spi without a CS pin.  It's now possible to
define multiple SPI messages to send on a shutdown event.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/atsamd/spi.c

@@ -6,9 +6,7 @@
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
 #include "internal.h" // enable_pclock
-#include "command.h" // shutdown
 #include "gpio.h" // spi_setup
-#include "sched.h" // sched_shutdown
 
 void
 spi_init(uint32_t bus, SercomSpi *ss, uint32_t ctrla, uint32_t baud)