pw_watch: Support specifying build targets
pw_watch: Support specifying build targets

- Allow specifying build directories and targets by appending #TARGET
  for each desired target.
- Provide the build commands as positional arguments instead of repeated
  --build_dir args.

Change-Id: Ia7631995941fec138ad1fd78d49db1e1f401a820

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