Add default build rules and reduce duplication am: 38e0d5b3b4 am: b11f334371 am: 863fb6f0c8

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

Change-Id: I30a964965d328f7850de4f3f1bce60d682980dc0
Signed-off-by: Automerger Merge Worker <[email protected]>