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.h

@@ -1,8 +1,10 @@
 #ifndef __BASECMD_H
 #define __BASECMD_H
 
+#include <stddef.h> // size_t
 #include <stdint.h> // uint8_t
 
+void *alloc_chunk(size_t size);
 void move_free(void *m);
 void *move_alloc(void);
 void move_request_size(int size);