Revert "dhclient: not always skip the nfsroot interface"

This reverts commit[27aec88c dhclient: not always skip
the nfsroot interface] which used to address the IP
address renew issue when boot a system in a nfsroot fs
and altogether boot with ip=dhcp.

But reported by some tester, the above commit introduces
below issue when run ltp test on a nfsroot system which
boot with ip=dhcp:
nfs: server 192.168.100.1 not responding, still trying
nfs: server 192.168.100.1 not responding, still trying
[snip]

So revert the above commit now to avoid blocking test.

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