update-alternatives: Remove build-time dependency

update-alternatives: Remove build-time dependency

Update-alternatives is actually a runtime dependency. The build-time
dependency is only needed to make sure update-alternative gets built
at all, because the runtime dependencies are generated too late for
bitbake to notice.

This breaks a dependency loop between dpkg and xz, if dpkg also
serves as the preferred runtime provider for update-alternatives.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>