PSA: drop `kernel.serial=acpi` from chromebook configurations

[Roland McGrath]

If you are using the chromebook-x64 board and have been injecting "kernel.serial=acpi" into your configuration, we recommend you stop doing that.

The boot shim used for chromebook images makes the default serial configuration be ACPI-based, but any explicit "kernel.serial=..." interferes with that.

The old "kernel.serial=acpi" will be going away now that the boot shim method removes the need for it.

If you don't have manual workflow adjustments involving an explicit kernel command line, then there is nothing to change.

The build is being changed in https://fuchsia-review.googlesource.com/c/fuchsia/+/556420 to stop embedding this string by default.