tentative fix: update hand-written Makefiles to use KBUILD_EXTMOD=

tentative fix: update hand-written Makefiles to use KBUILD_EXTMOD=

kernel 5.3+ deprecated and then removed the SUBDIRS=
flag for building an external module. In conjunction with KDIR=,
this led to unpredictable results (i.e. a build failure which
would delete the system kernel-devel package's config files (!)).

Commit 0126be38d9 counsels to use M= or KBUILD_EXTMOD= instead.
No harm in specifying both since newer kernels should ignore SUBDIRS=