systemd.bbclass: introduce systemd_service_searchpaths()

systemd.bbclass: introduce systemd_service_searchpaths()

systemd_service_searchpaths accepts boolean value indicating whether we
are dealing with system or user units and returns search paths
accordingly.

Previously search path list was created in systemd_check_services() but
following commits will introduce additional places. The
systemd_service_searchpaths helper function is meant to reduce code
duplication.

Signed-off-by: Artur Kowalski <arturkow2000@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>