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

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

Change-Id: I89e485649973c207df0879f00b67c7feab6dbba2
tree: 60cc9fcf841fd4e67ca7f3e3146342875a3925de
  1. devicetree/
  2. net/
  3. Android.bp
  4. METADATA