Thistle works with bootloaders in order to support root filesystem image update. Like all updates performed on the Thistle Platform, it is applied with an A/B policy - allowing to always fallback if an issue is encountered during the application of the update. Full system A/B boot process Multiple bootloaders are compatible with the Thistle Update Client. We provide reference implementations for Raspberry Pi 4, U-Boot, and for Grub If your platform uses a third party bootloader, feel free to explore the Custom bootloader.
As for any bootloader setup supported by the Thistle Update Client, it is required to have 2 partitions available, that will be used as root filesystem alternatively.
The 2 partitions used for A/B boot need to configured in the client configuration file config.json, see the example below. 
{
    ...
    "part_a": "/dev/sda",
    "part_b": "/dev/sdb",
    ...
}
Some bootloaders also require speficic configuration. Have a look at the dedicated bootloader pages for more information. In any case, feel free to reach out in our discord channel - we would be more than happy to hear about your bootloader setup and help with integration!