kernel.bbclass: make FILES package-specific

Fix QA warnings seen when using 'traditional' kernel recipes e.g.:

WARNING: QA Issue: .../recipes-kernel/linux/linux_3.0.18.bb: Variable
FILES is set as not being package specific, please fix this.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>