kbuild: fix 'make all install_modules install'

The command 'make all modules_install install' would fail
in a virgin tree - pointing at a non-existing directory under
/lib/modules/xxx

KERNELRELEASE is part of MODLIB and we need to create .kernelrelease
before we can properly evaluate KERNELRELEASE,
Changing MODLIB to the recursively expanded flavor let it pick up
the correct KERNELRELEASE value.

Reported by: "Hemmann, Volker Armin" <volker.armin.hemmann@tu-clausthal.de>

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