busybox: Enable domain search list support in udhcpc

busybox: Enable domain search list support in udhcpc

This is useful in networks with internal resources as it allows
to use much shorter names.

E.g. instead of "server.internal.company.com" it's possible
to use just "server" if DHCP server is configured with:
---------------------------->8-----------------------
option domain-search "internal.company.com";
---------------------------->8-----------------------

This improvement consists of 2 parts:

1. Enable handling of RFC3397 so DHCP client is ready for processing
   corresponding data from DHCP server.

2. Some DHCP servers always send out search list if it is set in server's
   configuration and some servers only provide search list if client
   asks for that (sending list of options it expects to get).

   And exactly for those stubborn DHCP servers we need to add "-O search"
   to udhcp's command line via CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Ignacy Gawedzki <ignacy.gawedzki@green-communications.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>