Resync with kbuild infrastructure
Our current build system is only synced with the Linux Kernel as of v4.20 and should be updated. In addition, there are some cleanups to what we have today that can be done, as noted by Yamada-san here: https://lore.kernel.org/all/CAK7LNAQbsaWV8abUzQS-XWB+mi+0uxy5fLvFJ=XPAmEPDUs_-Q@mail.gmail.com/