kernel.bbclass: Stop do_install poking directly into the sysroot and evading sstate

do_install was putting files directly into the sysroot which means sstate
had no knowledge of them. This meant they didn't get cleaned along with the
other files from the task amongst other issues.

This patch puts them in ${D} where they were supposted to be.

Tested-by: Tom Rini <tom_rini@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>