mini-lkh: mini linux kernel headers package

This package is intended for easy installation on systems that need a
minimal set of Linux kernel headers for building Busybox and/or low
level system tools, and don't want to deal with obtaining and
installing the official headers. They've been stripped down for
namespace cleanliness, omission of anything kernel-internal, and to
focus just on the interfaces where kernel headers are actually
necessary/useful (mainly special hardware access and configuration).

At present this package is highly incomplete. These headers were
originally packaged as part of musl libc, but I moved them to their
own dedicated repository to avoid potentially overwriting official
kernel headers when musl is upgraded.

These headers may be used without installation by adding the include
directory on the compiler's command line, or may be installed by
copying all the directories under include to a directory in the system
include search path (such as /usr/include).