build_rootfs.sh: Add 8250.nr_uarts for rockpi u-boot env am: 31f9aaf0d1 am: cec480c22e

Original change: https://android-review.googlesource.com/c/kernel/tests/+/2004904

Change-Id: I6bf7e38f441e521ad84fadd4183cd93438f03d55
tree: 60cc9fcf841fd4e67ca7f3e3146342875a3925de
  1. devicetree/
  2. net/
  3. Android.bp
  4. METADATA