api: Rename api_init() to legacy_api_init()

Complete the rename of this API so that it is clear it is considered
deprecated.

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

legacy_api/api.c

@@ -638,7 +638,7 @@ int syscall(int call, int *retval, ...)
 	return 1;
 }
 
-int api_init(void)
+int legacy_api_init(void)
 {
 	struct api_signature *sig;