debian: Use if/else rather than overriding default values
debian: Use if/else rather than overriding default values

This makes the code easier to follow.

Whilst here use DEB_HOST_ARCH_CPU for the ia64 and use the more
conventional syntax.

hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd