Same to
https://stackoverflow.com/questions/56926632/how-do-i-link-a-static-libra...
lib.a is create by "3.5 Library file goals - lib-y" see
"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation/kbuild/makefiles.rst".
Kernel version:5.0.9-301.fc30.x86_64
My makefile:
c-objs := b.o a.o libxxx.a libxx.a
obj-m := c.o