kernel.bbclass: test for hardlinks before installing

commit df564c4 [kernel: Use hardlinks for do_populate_sysroot for speed]
does indeed speed up the build, but it also means that the KERNEL_OUPUT
and KERNEL_IMAGETYPE may be hardlinks.

If they are hardlinks, install complains that they are actually the same
file and stops the build. We can easily test and avoid the copy if the
file is already in place.

[YOCTO #5527]

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>