abuild-rootbld: allow installing extra packages via config option

This all started with me wondering why pigz was not used for compressing generated tarballs with abuild rootbld even though I have it installed. As it turns out, pigz is (by default) not installed within the rootbld environment. In order to fix this, I propose a more general feature where people can specify extra packages that should be installed, within the environment created by abuild rootbld, via a configuration option.

Apart from pigz, I also deem this useful for install debugging tools e.g. strace or stuff like cmd:sha512sum for faster checksum generation.