kernel: avoid copying unnecessary files during do_install

kernel_do_install() populates $kerneldir with files needed to build
external modules. To accomplish this there are several copy commands
to get source from the kernel source tree and build trees after which
a 'clean' is performed. Since we are copying from the build tree we
get about 1G of .o and .cmd files copied over only to have them
removed when we clean. This adds additional IO overhead as well as
overhead caused by pseudo. By avoiding copying these files in the
first place we get multiple gains:
  * avoid initial copy
  * avoid file deletes during clean
  * reduce pseudo overhead

Additionally we are making use of cpio vs cp which tends to be
significantly faster at performing copies.

With these changes I observe a 15-30% decrease in the time to complete
the do_install() operation on the kernel.

[YOCTO #3517]

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

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