osdep: warn about osdep overriding each other only for this specific OS

This avoids having a warning if a package set has for instance added
the relevant osdeps for new operating systems that are not supported
by the original package set.
Merge pull request #14 from rock-core/restrict_osdep_overriding_warning_to_current_os

osdep: warn about osdep overriding each other only for this specific OS