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: Brian Rzycki <bmr@freescale.com>