kbuild: in makefile.txt note that Makefile is preferred name for kbuild files

As noted by Roland Dreier <rdreier@cisco.com> makefiles.txt told
one to use the name 'Kbuild' as preferred name for kbuild files.
This is not yet true so let makefiles.txt reflect reality.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>