With the new kernel versioning scheme, the first release in a series only has a version number and
a major revision number (e.g. linux 3.0, linux 3.1-rc1, etc.). Unfortunately, the module
directory for these kernels still has a minor revision number (e.g. lib/modules/3.0.0, lib/modules/3.0.1-rc1, etc.).
This causes problems for packages such as broadcom_sta that need to know the module directory, so
this attribute will allow setting the module directory version number separate from the
kernel version number when necessary.
svn path=/nixpkgs/trunk/; revision=28405
ffdc37215a