Add default build rules and reduce duplication am: 38e0d5b3b4 am: b11f334371

Original change: https://android-review.googlesource.com/c/platform/external/openwrt-prebuilts/+/2370400

Change-Id: I08fedbae2659eb313ebc55db5d828957db25c59e
Signed-off-by: Automerger Merge Worker <[email protected]>