openwrt: Store Kernel Debug-Info (#1971)

This change stores a Kernel with Debug-Symbols for the current
architecture in a new output directory '<outputdir>/debug'.
This allows a developer or operator of a network to store the kernel
along with the actual images. In case of a kernel oops the debug
information can be used with the script
'scripts/decode_stacktrace.sh' in the kernel source tree to get the
names to the symbols of the stack trace.

OpenWRT already provides the CONFIG_COLLECT_KERNEL_DEBUG -option that
creates a kernel with debug-symbols in the OpenWRT output directory.
This change enables this option and copies the generated kernel to the
gluon output directory.

Signed-off-by: Chrissi^ <chris@tinyhost.de>