[dhcp] Accept filename via DHCP option 67 as well as BOOTP filename field

Allow options with dedicated BOOTP fields to fall back to using the
equivalent DHCP option if the relevant field is empty.

src/net/dhcppkt.c

 		memset ( field_data, 0, field->len );
 		memcpy ( dhcp_packet_field ( dhcppkt->dhcphdr, field ),
 			 data, len );
+		/* Erase any equivalent option from the options block */
+		dhcpopt_store ( &dhcppkt->options, tag, NULL, 0 );
 		return 0;
 	}
 
 		    void *data, size_t len ) {
 	struct dhcp_packet_field *field;
 	void *field_data;
-	size_t field_len;
+	size_t field_len = 0;
 	
-	/* If this is a special field, return it */
+	/* Identify special field, if any */
 	if ( ( field = find_dhcp_packet_field ( tag ) ) != NULL ) {
 		field_data = dhcp_packet_field ( dhcppkt->dhcphdr, field );
 		field_len = field->used_len ( field_data, field->len );
-		if ( ! field_len )
-			return -ENOENT;
+	}
+
+	/* Return special field, if it exists and is populated */
+	if ( field_len ) {
 		if ( len > field_len )
 			len = field_len;
 		memcpy ( data, field_data, len );