Split network bringup out of base ramdisk init.

We now run the network bringup portion of the ramdisk init from an
init.d script, thus allowing image builders to inject code into the init
script before that point (e.g. configure network hardware).
The environment variables used by the network bringup code are retained
in the base init script so they can be overridden by init.d fragments.

Change-Id: I1ef0bb21e7f26c0ff3f02266f853ce5402bcb94d
Closes-Bug: #1252023
Merge "Split network bringup out of base ramdisk init."