esp_system, vfs: fix incomplete blocking reads in vfs_cdcacm

Blocking read from cdcacm VFS could return less bytes than requested.
This didn’t match the behaviour of other VFS drivers, and higher level
code could misbehave.

components/esp_system/include/esp_private/usb_console.h

@@ -1,16 +1,8 @@
-// Copyright 2019-2020 Espressif Systems (Shanghai) PTE LTD
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
+/*
+ * SPDX-FileCopyrightText: 2019-2022 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
 
 #pragma once
 
@@ -62,7 +54,7 @@ ssize_t esp_usb_console_flush(void);
 
 ssize_t esp_usb_console_read_buf(char* buf, size_t buf_size);
 
-bool esp_usb_console_read_available(void);
+ssize_t esp_usb_console_available_for_read(void);
 
 bool esp_usb_console_write_available(void);