busybox: add support for CHECKSUM_PARTIAL to udhcpc

When running as KVM or Xen guest, packets may be received with
incomplete checksum[1].  Patch adopted from Fedora dhcp package.

[1]. http://article.gmane.org/gmane.linux.kernel/1003853



Signed-off-by: Alexey I. Froloff <raorn@raorn.name>

SVN-Revision: 33475