Optimize RPM build process

Optimize RPM build process

* Remove unused variable OS_VERSION_ID
* Do all RPM building in build-root/rpmbuild
* Add build-root/rpmbuild to .gitignore
* Move redundant code to a target called 'spec'
* Copy spec file to SPECS instead of SOURCES
* Only set %{_topdir} macro when it's undefined, and because of that
  remove sed call to delete definition on spec file
* Pass _version and _release using --define parameter instead of using
  sed to add them to .spec
* Move rpm files to build-root after build finishes
* Add dist tarball to .gitignore

Change-Id: I230d60b62914056b353f7f8701d14754b0bf7f8c
Signed-off-by: Renato Botelho do Couto <renato@netgate.com>