kernel-devicetree: allow specification of dtb directory

kernel-devicetree: allow specification of dtb directory

Fedora/Redhat and Arch are somewhat standardized on their dtb directory
structure. Let's add some flags to configure yocto to mimic that
behavior.

Add the following variables to the kernel class:
	- KERNEL_DTBDEST (controls the destination directory for dtbs)
	- KERNEL_DTBVENDORED (controls if vendor subdirectories are to
	  be respected)

Currently KERNEL_DTBDEST is expected to be a subdir of KERNEL_IMAGEDEST
and KERNEL_DTBVENDORED is expected to be "true"/"false". This only
applies to the package directory structure. The deploydir structure is
purposely left untouched for compatibility with existing recipes.

By default this is configured to behave the same as the current recipe
and produce a flat dtb directory at KERNEL_IMAGEDEST.

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>