Fix deps for notice generation.

With "foo bar : req", the build command will be run separately for foo
and bar.
Instead with "foo : bar" and "bar : req", the build command will be run
only once to produce both targets.
Also running the same build command concurrently may lead to build race
condition.

Change-Id: I8af2817546a7e2697a553c9320bd10f6bd4f20a2