RFE: Enable autoboot strings by default

This is a feature request as opposed to a bug.

I would be super handy to have the following settings enabled by default in the Raspberry Pi Ubuntu u-boot package.

CONFIG_AUTOBOOT_KEYED
CONFIG_AUTOBOOT_DELAY_STR "delay"
CONFIG_AUTOBOOT_STOP_STR "stop"

The documentation for these settings is here: https://gitlab.denx.de/u-boot/custodians/u-boot-raspberrypi/-/blob/master/doc/README.autoboot

Many of us using Ubuntu on Raspberry Pi are using the serial ports for connecting various devices/hats. As Ubuntu currently ships, any character received on the serial port will halt the boot process. Devices, such as GPS receivers, typically have a regular stream of output that, when attached at boot, cause u-boot to halt.

By setting the above settings (or something similar) it will require a specific string to be entered to halt u-boot, instead of any character.

Reconfiguring u-boot to not abort on any character is quite hard and poorly documented.