Check 30 seconds for USB port

src/systemcmds/nshterm/nshterm.c

@@ -62,7 +62,9 @@ nshterm_main(int argc, char *argv[])
     }
     uint8_t retries = 0;
     int fd = -1;
-    while (retries < 50) {
+
+    /* try the first 30 seconds */
+    while (retries < 300) {
         /* the retries are to cope with the behaviour of /dev/ttyACM0 */
         /* which may not be ready immediately. */
         fd = open(argv[1], O_RDWR);