build: release.mk: further alignment with fellow projects

• reflect the project is also set to generate bzip2 tarball
  • this change should have been part of d95ee95
• ensure checksum file generated even w/o signing
• allow full-fledged release delivery testing incl. signing
• deal with GNU make not stripping, at non-empty value, trailing whitespace-to-comment-delimiter
  • these changes should have been part of a8a1d93

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>