google-x64cros: new device

This is my initial attempt at a generic port for x86_64 Chromebooks. It's mostly a copy of tablet-x64uefi with the addition of a ChromeOS kernel partition. I'm keeping the grub parts so it's still bootable with MrChromebox's firmware as noted in the wiki.

Unlike ARM boards the bootloader specifics don't seem to change much between x86 devices, but that's an educated guess at best as I only tested on one I have. There is a 8/16/32/512M size limit based on board's age. Currently the vmlinuz.kpart is at a nice ~10.5M. I set the kernel partition size to 16M to make sure things loudly break if we ever exceed that. (It would immediately jump to >70M so 32M doesn't make sense for that.)

Some issues:

• FDE with unl0kr doesn't boot at all. (Missing modules)
• Coreboot framebuffer doesn't work (maybe for FDE unlocking; or put i915 etc in initramfs-extra?)
• Choosing linux-lts as kernel somehow uses vmlinuz-edge in grub.cfg
• Suspend makes the firmware go into recovery, forces internal boot (Need to enable TPM drivers in kernels)
• There are reports of people damaging the speakers while tinkering with audio stuff on various Chromebooks (even ARM ones). I'm not sure if the default configuration is safe for all, mine happened to work fine.

There's a LOT of x86 Chromebooks, I'd appreciate more testing.