[paver][build] Split chromebook into a separate board from x64

As part of migrating towards a unified A/B/R boot image configuration
for all supported boards the following changes were made:
* A chromebook board file was added.
** VBOOT images are only built for the new target.
** VBOOT images are treated similarly to how signed images on other
   boards are treated.
* The EFI image was deduplicated into bootloader.
* The KERNC image was deduplicated into ZIRCON-A.
* Support for a VB_META_R partition was added.
* zedboot version was updated (due to breaking changes)

ZX-3861

Tested: Paved pixelbook, NUC, astro, and vim2
Change-Id: I2344048ef69dd7b5e53004c707a261f5e0e0bb93