Add default build rules and reduce duplication am: 38e0d5b3b4

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

Change-Id: Iaf60903cdab51be135290ff34569aa0bb77486c8
Signed-off-by: Automerger Merge Worker <[email protected]>