kernel: add missing path to search for debug files

Since explicit debug package creation via ${KERNEL_PACKAGE_NAME}-dbg has
been added to kernel, it has to cover all PACKAGE_DEBUG_SPLIT_STYLE
options. For ex. when the variable "debug-file-directory" package search
path has to be set explicitly, otherwise it will not find any files.

Signed-off-by: Andrej Valek <andrej.valek@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9c39da14)
Signed-off-by: Steve Sakoman <steve@sakoman.com>