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/basecmd.c

@@ -27,7 +27,7 @@ alloc_init(void)
 DECL_INIT(alloc_init);
 
 // Allocate an area of memory
-static void *
+void *
 alloc_chunk(size_t size)
 {
     if (alloc_end + size > dynmem_end())