pxe: Adding network configuration at boot time

When booting up the bootstrap, the current configuration was all
interfaces shall wait for a DHCP server.

In some cases, this could be a problem since you don't have DHCP or some
interfaces shall not be configured or get any DHCP lease.

To solve this, the IP= command allow defining for every interface its
configuration.

IP= expect a comma separated list of configurations.
Each configuration is made like:
        interface_name:configuration

interface_name is the "ethx" device name under Linux.
If interface_name is set to "all" it will means that all detected
network interfaces will be configured the same.

configuration could have the following values:
        none, dhcp, <CIDR>

none   : means that no ip shall be configured
dhcp   : means that a dhcp request shall be done on this interface
<CIDR> : mean we shall use a CIDR notation for the adress like "192.168.1.1/24".

Typical uses looks like :

make img IP=eth0:192.168.1.1/24,eth1:10.0.1.1/8,eth2:172.17.1.1/16

or

make img IP=eth0:192.168.1.1/24,eth1:dhcp,eth2:none

By default IP="all:dhcp" to insure the same behavior as prior the patch.

This IP= value can be set at compilation time but also at boot time
regarding your bootloader (PXE/USB/...)