control/Makefile.am - enhancements

- process the current directory first to ensure `control.rng` file is generated
  before it is used in the `examples` subdirectory (fixes `make check` failure
  when the RNG file does not exist yet)

- `make check` target:
  - removed duplicated `trang` command, create the RNG file via dependency
  - filter XML files from `$(control_DATA)` instead of hardcoding `control.xml` file
Merge pull request #60 from yast/makefile_rng_enhancements

control/Makefile.am - enhancements