I should have used 'M=' in (e93c53ce05 "Makefile: SUBDIRS ->
KBUILD_EXTMOD") originally.
See:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/kbuild/modules.rst?h=v5.3#n77
Signed-off-by: Shaun Ruffell <sruffell@sruffell.net>