Force statically configured eth mgmt interfaces up

Reviewed By: david_yang <david_yang@accton.com>

For development purposes you can statically configure the Ethernet
management interface by setting the "ip=" kernel command line option.
See kernel/linux/Documentation/filesystems/nfs/nfsroot.txt for more on
that.

ONIE had a bug where statically configured eth mgmt interfaces were
being left administratively down.

This patch puts the static configuration method on equal footing with
the other dynamic configuration methods, like the DHCPv4 and fall back
IP methods.  With this change, statically configured interfaces are
brought administratively up as part of the natural configuration flow.

Testing Done:

Built images for kvm_x86_64 and fsl_p2020rdbpca.

On x86 set the ip= kernel command line option from the GRUB menu.
Verified when the machine booted that eth0 was up and configured as
specified.

On PowerPC added the ip= kernel command line option using the
$onie_debugargs variable.  Then booted into ONIE rescue mode.
Verified when the machine booted that eth0 was up and configured as
specified.