vfs: add eventfd support

eventfd imitates the behavior of POSIX api `man(2) eventfd`. This api
can be used to pass events to a select based message loop.

components/vfs/include/esp_vfs_eventfd.h

@@ -0,0 +1,61 @@
+// Copyright 2021 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
+
+#pragma once
+
+#include <stddef.h>
+#include <sys/types.h>
+
+#include "esp_err.h"
+
+#define EFD_SUPPORT_ISR (1 << 4)
+#define EVENT_VFS_PREFIX  "/dev/event"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief  Registers the event vfs.
+ *
+ * @return  ESP_OK if successful, ESP_ERR_NO_MEM if too many VFSes are
+ *          registered.
+ */
+esp_err_t esp_vfs_eventfd_register(void);
+
+/**
+ * @brief  Unregisters the event vfs.
+ *
+ * @return ESP_OK if successful, ESP_ERR_INVALID_STATE if VFS for given prefix
+ *         hasn't been registered
+ */
+esp_err_t esp_vfs_eventfd_unregister(void);
+
+/*
+ * @brief Creates an event file descirptor.
+ *
+ * The behavior of read, write and select is the same as man(2) eventfd with
+ * EFD_SEMAPHORE **NOT** specified. A new flag EFD_SUPPORT_ISR has been added.
+ * This flag is required to write to event fds in interrupt handlers. Accessing
+ * the control blocks of event fds with EFD_SUPPORT_ISR will cause interrupts to
+ * be temporarily blocked (e.g. during read, write and beginning and ending of
+ * the * select).
+ *
+ * @return The file descriptor if successful, -1 if error happens.
+ */
+int eventfd(unsigned int initval, int flags);
+
+#ifdef __cplusplus
+}
+#endif