busybox: Support option 119 in udhcpc default script.

busybox: Support option 119 in udhcpc default script.

* package/busybox/udhcpc.script: Use $search instead of $domain if
  that is defined.

Note that for this to work, not only has CONFIG_FEATURE_UDHCP_RFC3397
be enabled in Busybox' configuration but also the option has to be
requested using the "-O search" command-line option (one can set it in
CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS, for instance).

[Peter: add comment with reference to rfc3359/option 119 for clarity]
Signed-off-by: Ignacy Gawędzki <ignacy.gawedzki@green-communications.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>