nshlib/md5: Support reading from standard input

Sometimes users may want to calculate the MD5 of part of a file(e.g. check
partition contents for debug after flashing, size of which may be greater
than image). This can be done with the "dd" command and pipes, the "md5"
command just needs to support reading from standard input. For example:
`dd if=/dev/virtblk0 bs=512 count=1 | md5`.

Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

nshlib/nsh_codeccmd.c

@@ -509,6 +509,26 @@ int cmd_md5(FAR struct nsh_vtbl_s *vtbl, int argc, FAR char **argv)
           return ret;
         }
     }
+  else if (argc == 1)
+    {
+      MD5_CTX ctx;
+
+      md5_init(&ctx);
+
+      while (1)
+        {
+          ret = nsh_read(vtbl, digest, sizeof(digest));
+          if (ret <= 0)
+            {
+              ret = ret < 0 ? -errno : 0;
+              break;
+            }
+
+          md5_update(&ctx, digest, ret);
+        }
+
+      md5_final(digest, &ctx);
+    }
   else
     {
       md5_sum((FAR unsigned char *)argv[1], strlen(argv[1]), digest);