build_package: fix loop between two loop breaking steps

If util-linux has a binary package it will be used, but if that binary
package has +udev it will pull in systemd. systemd has a loop that needs
to be broken too so if the util-linux loop breaker doesn't also handle
the systemd one it all falls apart.

In short the comment above the loop breaker code noting that we can try
this until it gets wonky. Well, it is wonky and we need to re-do how
build_packages works as a result. This is just a temporary workaround
until we figure out a larger restructuring.
Merge pull request #307 from marineam/fix-loop

build_package: fix loop between two loop breaking steps