For now this is a raw copy from the old Documentation/kbuild/modules.txt,
which was removed in 2.6.0-test5.
The information herein is correct but not complete.

Installing modules in a non-standard location
---------------------------------------------
When the modules needs to be installed under another directory
the INSTALL_MOD_PATH can be used to prefix "/lib/modules" as seen
in the following example:

make INSTALL_MOD_PATH=/frodo modules_install

This will install the modules in the directory /frodo/lib/modules.
/frodo can be a NFS mounted filesystem on another machine, allowing
out-of-the-box support for installation on remote machines.


Compiling modules outside the official kernel
---------------------------------------------
Often modules are developed outside the official kernel.
To keep up with changes in the build system the most portable way
to compile a module outside the kernel is to use the following command-line:

make -C path/to/kernel/src SUBDIRS=$PWD modules

This requires that a makefile exits made in accordance to
Documentation/kbuild/makefiles.txt.

