esp32/network_ppp: Allow building with IPv6 disabled.

PPP code assumes that IPv6 support is enabled.  Whilst this is the default,
certain applications may want to disable IPv6 support if not needed (or to
reduce code size).

This makes the code build with CONFIG_LWIP_IPV6 disabled, reducing code by
about 30k in that case.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

ports/esp32/network_ppp.c

@@ -65,7 +65,11 @@ static void ppp_status_cb(ppp_pcb *pcb, int err_code, void *ctx) {
 
     switch (err_code) {
         case PPPERR_NONE:
+            #if CONFIG_LWIP_IPV6
             self->connected = (pppif->ip_addr.u_addr.ip4.addr != 0);
+            #else
+            self->connected = (pppif->ip_addr.addr != 0);
+            #endif // CONFIG_LWIP_IPV6
             break;
         case PPPERR_USER:
             self->clean_close = true;
@@ -250,7 +254,11 @@ STATIC mp_obj_t ppp_ifconfig(size_t n_args, const mp_obj_t *args) {
         ip_addr_t dns;
         mp_obj_t *items;
         mp_obj_get_array_fixed_n(args[1], 4, &items);
+        #if CONFIG_LWIP_IPV6
         netutils_parse_ipv4_addr(items[3], (uint8_t *)&dns.u_addr.ip4, NETUTILS_BIG);
+        #else
+        netutils_parse_ipv4_addr(items[3], (uint8_t *)&dns, NETUTILS_BIG);
+        #endif // CONFIG_LWIP_IPV6
         dns_setserver(0, &dns);
         return mp_const_none;
     }