ifupdown: fix issue for configuring loopback interface

ifupdown: fix issue for configuring loopback interface

The patch inet-6-.defn-fix-inverted-checks-for-loopback.patch introduced
an issue that ifup/ifdown can not configure loopback interface:

$ cat /etc/network/interfaces
auto lo
iface lo inet loopback

$ ifconfig -s
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500 0         1      0      0 0            74      0      0 0 BMRU
lo        65536 0         2      0      0 0             2      0      0 0 LRU
$ ifdown lo
$ ifconfig -s
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500 0         1      0      0 0            74      0      0 0 BMRU
lo        65536 0         2      0      0 0             2      0      0 0 LRU

The original patch is for 0.7.x and the change is not needed for 0.8.x.
Update the patch to make the ifup/ifdown can configure lo interface:
$ ifconfig -s
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500 0         0      0      0 0            77      0      0 0 BMRU
lo        65536 0         2      0      0 0             2      0      0 0 LRU
$ ifdown lo
$ ifconfig -s
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500 0         0      0      0 0            80      0      0 0 BMRU
$ ifup lo
$ ifconfig -s
Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0       1500 0         0      0      0 0            81      0      0 0 BMRU
lo        65536 0         2      0      0 0             2      0      0 0 LRU

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>