[FEAT] Support tryboot helpers without ramdisk

[bryceschober]

I'm digesting the implications of the tryboot_a_b update flow, and it's becoming clear that it is more designed for boot systems that depend on a ramdisk. Since the ramdisk would be named the same on the A/B boot partitions, it doesn't need any differences in cmdline.txt, and init logic within it can then use the device tree's chosen/bootloader properties to drive its dynamic logic.

But for systems that would prefer not to have a separate ramdisk, they are currently required to modify their kernel cmdline appropriately for the A/B partition that is being booted, and set up for that during their update process along with modifying the autoboot.txt.

I can think of two possible ways that the rpi firmware could support this use-case:

1. Support the cmdline setting in the autoboot.txt in addition to the config.txt. This way, the A/B boot partitions could be identical and contain both cmdline_A.txt and cmdline_B.txt files with the appropriate root= values.
2. Support a [boot_partition=n] filter in config.txt that would be filled by the active configuration from the autoboot.txt logic. This way, the config.txt could conditionally point to an appropriate cmdline*.txt next to it.