net: Quietly ignore DHCP Option 28 (Broadcast Address)

Some DHCP servers (notably dnsmasq) always transmit DHCP Option 28,
Broadcast Address as specified in RFC 2132. Without this patch u-boot
displays the warning:
  *** Unhandled DHCP Option in OFFER/ACK: 28

The patch suppresses the warning and ignores DHCP Option 28. There is
no environment variable to set the broadcast address into and if for
some reason u-boot needs the broadcast it can be calculated from
ipaddr and netmask.
Signed-off-by: NBrian Rzycki <bmr@freescale.com>

net/bootp.c

@@ -728,6 +728,8 @@ static void DhcpOptionsProcess(uchar *popt, struct Bootp_t *bp)
 			memcpy(&NetOurRootPath, popt + 2, size);
 			NetOurRootPath[size] = 0;
 			break;
+		case 28:	/* Ignore Broadcast Address Option */
+			break;
 #if defined(CONFIG_CMD_SNTP) && defined(CONFIG_BOOTP_NTPSERVER)
 		case 42:	/* NTP server IP */
 			NetCopyIP(&NetNtpServerIP, (popt + 2));