kernel: Use hardlinks during do_install for speed

kernel: Use hardlinks during do_install for speed

Whilst the comment says we can't do this, its incorrect, install operates
on WORKDIR and its hard for different parts of WORKDIR to be on different
filesystems. Hardlinking instead of copying is therefore a nice performance
gain.

Also, completely skip the Documentation directory (adding a dummy Makefile
to keep make happy) and tweak the other cp commands to link since if we
don't we'd get "this is the same file" type errors from cp for some kernel
versions.

For do_install on linux-yocto, this takes it from 227s -> 84s.

(From OE-Core rev: 0727e510f8533216518563b1533e804a1dc44402)

(From OE-Core rev: f6e98af6b9f6729bb61f17e3b4203c3c79829205)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>