Eliminate targets from rpmBuildArguments

Eliminate targets from rpmBuildArguments
- Targets are obviously relevant for building but not in the
  context of rpmSpecBuild which expects the environment to be set up
  already. If/when we add a higher level build interface to librpmbuild
  this comes relevant again, until then bury inside the cli-tool.
- Simplify the target parsery by using argvSplit() and friends.