systemd: Consider PACKAGECONFIG in RRECOMMENDS

Since RRECOMMENDS declaration implictly induces building the recipes
that provide the runtime recommended packages, conditionalize adding
such values according to associated PACKAGECONFIG settings in order
to avoid redundant building.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>