feat: 新增musl c库的realpath实现

切换newlibc的时候，realpath的实现也从这里实现。
Signed-off-by: NSimonLi <likailong@huawei.com>

porting/liteos_m/kernel/BUILD.gn

@@ -59,6 +59,7 @@ static_library(libc) {
     "src/locale/c_locale.c",
     "src/locale/langinfo.c",
     "src/misc/dirname.c",
+    "src/misc/realpath.c",
     "src/multibyte/internal.c",
     "src/multibyte/mbtowc.c",
     "src/multibyte/wcrtomb.c",

porting/liteos_m/kernel/src/misc/realpath.c

+/*
+ * Copyright (c) 2013-2019 Huawei Technologies Co., Ltd. All rights reserved.
+ * Copyright (c) 2020-2021 Huawei Device Co., Ltd. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without modification,
+ * are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this list of
+ *    conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice, this list
+ *    of conditions and the following disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ *
+ * 3. Neither the name of the copyright holder nor the names of its contributors may be used
+ *    to endorse or promote products derived from this software without specific prior written
+ *    permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+ * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+ * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <limits.h>
+#include <errno.h>
+#include <sys/stat.h>
+
+/*
+ * LiteOS-M file system uses abs path
+ */
+
+static unsigned int vfs_strnlen(const char *str, size_t maxlen)
+{
+    const char *p = NULL;
+
+    for (p = str; ((maxlen-- != 0) && (*p != '\0')); ++p) {}
+
+    return p - str;
+}
+
+char *realpath(const char *__restrict path, char *__restrict resolved_path)
+{
+    int namelen;
+    int res;
+    struct stat st;
+
+    if (!*path) {
+        errno = ENOENT;
+        return NULL;
+    }
+
+    namelen = vfs_strnlen(path, PATH_MAX);
+    if (!namelen) {
+        errno = EINVAL;
+        return NULL;
+    } else if (namelen >= PATH_MAX) {
+        errno = ENAMETOOLONG;
+        return NULL;
+    }
+
+    // LiteOS-M file system uses abs path, so we just stat path and strcpy it.
+    if (stat(path, &st) < 0) {
+        errno = EINVAL;
+        return NULL;
+    }
+
+    res = strcpy_s(resolved_path, PATH_MAX, path);
+    if (res != 0) {
+        errno = EINVAL;
+        return NULL;
+    } else {
+        return resolved_path;
+    }
+}