systemd: add RRECOMMENDS for weak dependencies, if enabled

Some libraries are now dlopen'ed and skipped at runtime if
not available. Add them to the RRECOMMENDS list, to allow
users to exclude them for smaller images even if generally
enabled at build time.

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