ThinkPad

if already flashed, one can use flashprog -p internal. on the stock bios, at most you may be able to hack together a method to flash coreboot, but you can't neuter the ME, nor recover the megabytes of flash the ME firmware uses.

not sure about the t540p, but the t440p isn't too hard to externally flash. two screws to get inside, then the various internal screws (most of which are identical), remove the keyboard & trackpad, the VGA port screws, and then your flash chips are exposed. hook up an SOIC8 clip (on ebay for ~$5) to a pi pico, flash pico-serprog, and then use flashprog on another machine to flash the chips.

once successfully flashed, you can easily update coreboot from within the OS. however, if you flash a broken build, -- whether it's a lack of vendor blobs, like for the (neutered) ME, or simply a BIOS that can't get you to your OS (or a rescue USB) -- you will need to reflash externally before the machine will be usable again.