functions net_intf(): Correct undefined variables, $dhcp and $ip

Accton AS6700_32X has 2 mgmt ports.  If specify static IP for eth1
via ``onie_debugargs``, ONIE still tries to configure eth0.

The test procedure is:

1. The outer mgmt port plugs cable whose link partner has DHCP
   server actived.
2. The inner mgmt port does not plug any cable.
3. Power on the box. Set setenv onie_debugargs
   ``ip=192.168.6.10:::255.255.255.0::eth1`` and then
   ``run onie_bootcmd``.

ONIE will show the following messages and try to run DHCPv4 discover
on 'eth0'.

    ONIE: eth0: Using static IP config: ip=192.168.6.10:::255.255.255.0::eth1
    Info: eth1:  Checking link... down.
    ONIE: eth1: link down.  Skipping configuration.
    ONIE: Failed to configure eth1 interface
    ONIE: Starting ONIE Service Discovery

According to the log, it showes ONIE trying to discover images via
eth0.  But eth0 was not configured IP (from static IP or DHCP).

    ONIE: Trying DHCPv4 on interface: eth0
    ...
    Info: Fetching tftp://192.168.1.99/onie-installer-accton_as6700_32x ...
    Info: Fetching tftp://192.168.1.99/onie-installer-powerpc ...
    Info: Fetching tftp://192.168.1.99/onie-installer ...
    ...

The ``net_intf()`` reporting Ethernet interfaces depends on some
conditions.  If ``$dhcp`` or ``$ip`` is existent, it will try to
report the specified interface. Otherwise, it reports all Ethernet
interfaces on the box.

We found ``$dhcp`` and ``$ip`` are undefined here. They should be
``$onie_dhcp`` and ``$onie_ip``.

After fixing the issue.  ONIE handles the correct interface by the
same test procedure:

    Starting: dropbear ssh daemon... done.
    Starting: telnetd... done.
    discover: installer mode detected.  Running installer.
    Starting: discover... done.

    Please press Enter to activate this console. Info: eth1:  Checking link... down.
    ONIE: eth1: link down.  Skipping configuration.
    ONIE: Failed to configure eth1 interface
    ONIE: Starting ONIE Service Discovery
    Info: Sleeping for 20 seconds
    4..3..2..1..
discover: Calling import_cmdline() before calling net_intf()

In /rootconf/default/bin/discover, it uses ``net_intf()`` to get the
valid Ethernet interface list.  ``net_intf()`` uses ``$onie_dhcp`` and
``$onie_ip`` as conditions to generate the list.  The script should
explicitly call ``import_cmdline()`` to export these variables before
calling ``net_intf()``.