[coreboot] Makefile classes-y vs subdirs-y

[Maximilian Brune]

I am currently working a little bit on on makefile stuff and got a bit
confused.
Looking at the documentation it seems like `subdirs` is one of the possible
`classes` that we have.

But then I looked at payloads/libpayload/Makefile.mk file at this location:
https://elixir.bootlin.com/coreboot/latest/source/payloads/libpayload/Makefi...
and it seems to add classes where I would have expected it to use subdirs-y
instead.

Can someone explain to me when to use classes-y and when to use subdirs-y?