x86, kbuild: make "make install" not depend on vmlinux

It is common to use "make install" in restricted environments which
differ from the one which was actually used to build the kernel.  In
such environments it is highly undesirable to trigger a rebuild of any
part of the system.  Worse, the rebuild may be spurious, triggered by
differences in the environment.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <20090415234642.GA28531@uranus.ravnborg.org>