With reference to #287 and #288, we have the stamp flag supported in rpm_pfg, but one possible use case of stamping RPMs is to control the release string.

I'd propose that we allow {VARIABLE_NAME} placeholders in the release attribute of pkg_rpm. When stamp = 1 (or stamp = -1 with --stamp), workspace status variables from volatile-status.txt are substituted at build time.

This is very loosely analagous to the use of get_timestamp() in build_zip.py and build_tar.py.