dhclient: not always skip the nfsroot interface

Don't skip the nfsroot interface when use dhcp
to get the address for nfsroot interface as the
nfsroot interface may need dhclient to renew
the lease.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>