systemd: add usrmerge to REQUIRED_DISTRO_FEATURES

Support for unmerged-usr is deprecated upstream, taints the system and
has been removed for v255 (next release).
Enforce building merged-usr images when using systemd. This allows one
release cycle where it can be tested for any remaining issue, and can
still be overridden, before it stops working completely.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>