classes: Generalize construction of ros_prefix

In special situations, the prefix used by yocto is prefixed by an
additional staging dir and thus not always starting at /. The current
class already covered the special case for building native packages, but
was erroneous when trying to build nativesdk packages.

Instead of dealing with these special cases ourselves, we can prefix the
ros_prefix with yocto's base_prefix. This prefix will contain any
additional dirs yocto decides to use and will therefore always result in
valid directory variables automatically.