kernel: Move Device Tree support to kernel.bbclass
The Device Tree is commonly used but it is still kept as a .inc file
instead of a proper class. Instead now we move the Device Tree code to
a kernel-devicetree class and automatically enable it when the
KERNEL_DEVICETREE variable is set.
To avoid breakage in existing layers, we kept a linux-dtb.inc file
which raises a warning telling the user about the change so in next
release this can be removed.
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
