Issue: SLIP interface lacks a layer-2 address, breaking IPv6 link‑local and auto‑configuration

Issue: SLIP interface lacks a layer-2 address, breaking IPv6 link‑local and auto‑configuration: a task in LegoFlow-SWE (Harbor dataset). Symptom: When a SLIP network interface is used, the GNRC stack does not assign a layer‑2 (hardware) address to the interface ( NETOPT ADDRESS is not reported)…

The task

**Symptom:** When a SLIP network interface is used, the GNRC stack does not assign a layer‑2 (hardware) address to the interface (`NETOPT_ADDRESS` is not reported). Consequently, the flag `GNRC_NETIF_FLAGS_HAS_L2ADDR` is never set. This causes multiple problems:

Part of Lego-X/LegoFlow-SWE.