linux-dtb: Use kernel build system to generate the dtb files

linux-dtb: Use kernel build system to generate the dtb files

As the Linux kernel, unconditionally, builds the dtc application and
it is the compatible version with the DeviceTree files shipped within
the kernel it is better to use it and the kernel build system to
generate the dtb files.

Some DeviceTree files rely on CPP and kernel headers to be able to
generate the dtb binary contents and it is harder to replicate it
outside of Linux kernel build system so we /use/ it.

To comply with these assumptions we need to use the dtb file when
calling 'make' instead of pointing to the DeviceTree source file; the
code has been made backward compatible but it is advised to move to
the new definition to avoid warnings as:

,----[ Original definition ]
| KERNEL_DEVICETREE = "${S}/arch/arm/boot/dts/imx6q-sabresd.dts"
`----

Becomes:

,----[ New definition ]
| KERNEL_DEVICETREE = "imx6q-sabresd.dtb"
`----

(From OE-Core rev: 72980d5bb465f0640ed451d1ebb9c5d2a210ad0c)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>