lldpd: get_config_cid_ifaces() -> get_interface_csv()

lldpd: get_config_cid_ifaces() -> get_interface_csv()

where csv = comma separated value(s)

Make the function more generic. Can use it for not only 'config'.

Now it can be used to parse interfaces for additional lldpd settings,
e.g. custom-tlv.

Tested on: 22.03.6

Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/14872
(cherry picked from commit a015f59880c72a1bdd07de32491f85ce7f8d2a1c)
Link: https://github.com/openwrt/openwrt/pull/18343
Signed-off-by: Robert Marko <robimarko@gmail.com>