mkimage: Refactor imagetool_get_source_date to take command name

So we can use imagetool_get_source_date() from callers who do not have
the image tool params struct, just pass in the command name for the error
message.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromum.org>

tools/imagetool.h

@@ -216,12 +216,12 @@ int imagetool_get_filesize(struct image_tool_params *params, const char *fname);
  * an error message if SOURCE_DATE_EPOCH contains an invalid value and returns
  * 0.
  *
- * @params:	mkimage parameters
+ * @cmdname:	command name
  * @fallback:	timestamp to use if SOURCE_DATE_EPOCH isn't set
  * @return timestamp based on SOURCE_DATE_EPOCH
  */
 time_t imagetool_get_source_date(
-	struct image_tool_params *params,
+	const char *cmdname,
 	time_t fallback);
 
 /*