ipq40xx: Support Chromium OS image-type creation

ipq40xx: Support Chromium OS image-type creation

See firmware-utils.git commits [1], which implemented the cros-vbutil
verified-boot payload-packing tool, and extended ptgen for the CrOS
kernel partition type. With these, it's now possible to package kernel +
rootfs to make disk images that can boot a Chrome OS-based system (e.g.,
Chromebooks, or even a few AP models).

Regarding PARTUUID= changes: Chromium bootloaders work well with a
partition number offset (i.e., relative to the kernel partition), so
we'll be using a slightly different root UUID line.

NB: I've made this support specific to ip40xx for now, because I only
plan to support an IPQ4019-based AP that uses a Chromium-based
bootloader, but this image format can be used for essentially any
Chromebook, as well as the Google OnHub, a prior Chromium-based AP using
an IPQ8064 chipset.

[1]
ptgen: add Chromium OS kernel partition support
https://git.openwrt.org/?p=project/firmware-utils.git;a=commit;h=6c95945b5de973026dc6f52eb088d0943efa96bb

cros-vbutil: add Chrome OS vboot kernel-signing utility
https://git.openwrt.org/?p=project/firmware-utils.git;a=commit;h=8e7274e02fdc6f2cb61b415d6e5b2e1c7e977aa1

Signed-off-by: Brian Norris <computersforpeace@gmail.com>