busybox: udhcpc source IP rebind patch

Patch sets the source IP address of DHCP request messages during rebind to the IP address
assigned to the udhcpc client. Source address 0.0.0.0 can only be used by a client prior
to obtaining its IP address (see RFC2131 § 4.1). Source IP address behavior lines up now with
the ISC dhcp client implementation for DHCP request messages during rebind and DHCP release
messages.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40878 3c298f89-4303-0410-b956-a3cf2f4a3e73