pw_build: Fix target set_defaults() compatibility
pw_build: Fix target set_defaults() compatibility

A project may set default configs on targets such as source_set,
executable. This currently causes an error due to assigning a nonempty
list. Fix it by adding pigweed configs instead of assigning.

Change-Id: I06f7175c976c976f9505b6523141b2f280f50ff6

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed