This centralizes/factorizes the rpm package builds, and builds
RPM packages in the VM instead of on the host, which is cleaner. It also
paves the way for supporting Debian and arch builds.
Drop the `make srpm` rule, as it's not very useful. Keep the `make rpm`
rule, as sometimes developers do this manually. This might be replaced
later on with another image-customize feature which copies the built rpm
out of the VM.
Rename some files and change some identifiers to be neutral to the
application name. This makes it simpler to change everything to a proper
name when cloning this project.
Document in the README how to find the remaining places to change.
Closes#20
