[disk-pave] Add support for paving bootloader.

Currently only enabled for Astro and Gauss.

Change-Id: I5f64795039bbeaaf359b05ff74fc889e6c19d782
Tested: Paved bootloader to Astro.

[disk-pave] Add support for ARM platforms to paver

A new device-partitioner implementation was added that doesn't expect a
GPT was added. It does not have the ability to remove partitions or add
them, but rather only find them.

In addition, the new paver implementation works with the ZIRCON-A/B/R
partition layout instead of EFI/KERN-C partitions that ChromeOS and EFI
based systems expect.

Tested:
Manually tested paving on pixelbook and vim2.

Change-Id: I49005a8376e00fa034bfc77b7c0ffe752712207b

[disk-pave] Rewrite paver

An additional abstraction layer of a "device partitioner" interface was
added to the paver to allow for non-GPT device implementation to be
added more easily.

Tested:
Manually tested paving on pixelbook and nuc.
Tested wipe command followed by paving.
Tested paving to same device twice in a row.

Change-Id: I6b0d79c4702e20342e60125d15832e7fbb0b39fe