sandbox: log: Add a category for sandbox

It seems useful to make sandbox its own log category since it is used for
so much testing. Add this as a new category.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/log.h

@@ -47,6 +47,7 @@ enum log_category_t {
 	LOGC_DT,	/* Device-tree */
 	LOGC_EFI,	/* EFI implementation */
 	LOGC_ALLOC,	/* Memory allocation */
+	LOGC_SANDBOX,	/* Related to the sandbox board */
 
 	LOGC_COUNT,	/* Number of log categories */
 	LOGC_END,	/* Sentinel value for a list of log categories */