Net install changes.
* Got rid of confirmation step that required you to press <Space> to
initiate net install. Now just long press <Shift>
* Updated the screen text to make it more obvious the device is still
trying boot when the net install is showing.
* Fixed a DHCP net install bug which caused us to lose the
gateway address.
* Fixed a bug with the uIP timers which could cause net install to
always fail.
* Implemented resume and retry on download failure.
Other interesting changes.
* Allow smaller MSD discovery timeouts to be specified.
* Some tweaks and fixes to IPV6 netboot.
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