busybox: using ip instead of ifconfig for ifup/ifdown

There is an issue for requesting dynamic IP with ifup/ifdown command when using dhclient.

Steps to reproduce:
1. Build a full-cmdline image and install dhcp-client as the default DHCP client.
2. Configure a static IP for eth0 in /etc/networking/interfaces and reboot.

    $ ifconfig eth0
    eth0      inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0
    $ ifdown eth0
3. Modify /etc/networking/interfaces to configure a dynamic IP for eth0
    $ ifup eth0
    $ ifconfig eth0
    eth0      inet addr:192.168.1.2  Bcast:192.168.1.255  Mask:255.255.255.0

You could see the eth0 still has a static IP. But actually it also has a dynamic IP:

    $ ip addr show eth0
    eth0:
        inet 192.168.1.2/24 brd 192.168.1.255 scope global eth0
        inet 128.224.162.173/23 brd 128.224.163.255 scope global eth0

The root cause is the ifdown invokes "ifconfig" to down the eth0 but doesn't remove its IP. The dhclient would invoke "ip" to configure the interface. It can not remove an IP from down interface with "ip addr flush" and "ip addr add" command can set multiple IPs on one interface.

To fix this issue, we should use the "ip" command to implement ifup/ifdown, rather than using the older "ifconfig". It will flush the IP before down the interface.

---
 meta/recipes-core/busybox/busybox/defconfig | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

index 54202ef..cc68bea 100644
@@ -820,9 +820,9 @@ CONFIG_FEATURE_IFCONFIG_HW=y
 # CONFIG_IFPLUGD is not set
 CONFIG_IFUPDOWN=y
 CONFIG_IFUPDOWN_IFSTATE_PATH="/var/run/ifstate"
-# CONFIG_FEATURE_IFUPDOWN_IP is not set
-# CONFIG_FEATURE_IFUPDOWN_IP_BUILTIN is not set
-CONFIG_FEATURE_IFUPDOWN_IFCONFIG_BUILTIN=y
+CONFIG_FEATURE_IFUPDOWN_IP=y
+CONFIG_FEATURE_IFUPDOWN_IP_BUILTIN=y
+# CONFIG_FEATURE_IFUPDOWN_IFCONFIG_BUILTIN is not set
 CONFIG_FEATURE_IFUPDOWN_IPV4=y
 CONFIG_FEATURE_IFUPDOWN_IPV6=y
 CONFIG_FEATURE_IFUPDOWN_MAPPING=y

2.7.4