support elfloader for shared object and relocatable object

1. elfloader for shared object
example: TencentOS-tiny\examples\elfloader_shared_object
keil: TencentOS-tiny\board\TencentOS_tiny_EVB_MX_Plus\KEIL\elfloader_shared_object

2. elfloader for relocatable object:
example: TencentOS-tiny\examples\elfloader_relocatable_object
keil: TencentOS-tiny\board\TencentOS_tiny_EVB_MX_Plus\KEIL\elfloader_relocatable_object

3. TODO:
- add icache/dcache flush when module is loaded
- support more relocation type in elfloader_arch_relocate

components/elfloader/common/tos_elfloader_fd_read-vfs.c

@@ -0,0 +1,40 @@
+/*----------------------------------------------------------------------------
+ * Tencent is pleased to support the open source community by making TencentOS
+ * available.
+ *
+ * Copyright (C) 2019 THL A29 Limited, a Tencent company. All rights reserved.
+ * If you have downloaded a copy of the TencentOS binary from Tencent, please
+ * note that the TencentOS binary is licensed under the BSD 3-Clause License.
+ *
+ * If you have downloaded a copy of the TencentOS source code from Tencent,
+ * please note that TencentOS source code is licensed under the BSD 3-Clause
+ * License, except for the third-party components listed below which are
+ * subject to different license terms. Your integration of TencentOS into your
+ * own projects may require compliance with the BSD 3-Clause License, as well
+ * as the other licenses applicable to the third-party components included
+ * within TencentOS.
+ *---------------------------------------------------------------------------*/
+
+#include "tos_vfs.h"
+#include "tos_elfloader.h"
+
+/*
+ ATTENTION:
+    if you wanna load the so/obj laying on your flash, you should implement "elfloader_fd_read"
+    with flash driver read/write operation.
+
+    very easy to use whether your module is on a file system or raw flash.
+ */
+__KNL__ __WEAK__ el_err_t elfloader_fd_read(int fd, uint32_t offset, void *buf, size_t len)
+{
+    if (tos_vfs_lseek(fd, (vfs_off_t)offset, VFS_SEEK_SET) < 0) {
+        return ELFLOADER_ERR_FD_READ_FAILED;
+    }
+
+    if (tos_vfs_read(fd, buf, len) < 0) {
+        return ELFLOADER_ERR_FD_READ_FAILED;
+    }
+
+    return ELFLOADER_ERR_NONE;
+}
+