For tiny targtes, LWIP_RAND is optional -> fix compile time checks

(cherry picked from commit 693a74c286c3712a5a8e5ed99f6f6b82ed43a080)

src/core/init.c

@@ -218,9 +218,6 @@ PACK_STRUCT_END
 #if !LWIP_ETHERNET && (LWIP_ARP || PPPOE_SUPPORT)
   #error "LWIP_ETHERNET needs to be turned on for LWIP_ARP or PPPOE_SUPPORT"
 #endif
-#if (LWIP_IGMP || LWIP_IPV6) && !defined(LWIP_RAND)
-  #error "When using IGMP or IPv6, LWIP_RAND() needs to be defined to a random-function returning an u32_t random value (in arch/cc.h)"
-#endif
 #if LWIP_TCPIP_CORE_LOCKING_INPUT && !LWIP_TCPIP_CORE_LOCKING
   #error "When using LWIP_TCPIP_CORE_LOCKING_INPUT, LWIP_TCPIP_CORE_LOCKING must be enabled, too"
 #endif