core/dhcp: Add check to ensure DHCP cannot be used without IP broadca…

…st support

liteeth/core/__init__.py

@@ -93,6 +93,8 @@ def __init__(self, phy, mac_address, ip_address, clk_freq, arp_entries=1, dw=8,
     ):
         # Ensure either IP is external or DHCP is used
         assert((ip_address is None) == with_dhcp)
+        # DHCP requres IP broadcast
+        assert(not with_dhcp or with_ip_broadcast)
 
         # Parameters.
         # -----------