Rework machines to use dtb file in KERNEL_DEVICETREE variable

Rework machines to use dtb file in KERNEL_DEVICETREE variable

The linux-dtb.inc has been reworked to use the Linux kernel build
system to generate the DeviceTree binaries so now KERNEL_DEVICETREE
variable should has the /target/ name, not the file path. This patch
reworks following machines:

 - imx23evk
 - imx28evk
 - imx51evk
 - imx53ard
 - imx53qsb
 - imx6dlsabreauto
 - imx6dlsabresd
 - imx6qsabreauto
 - imx6qsabresd
 - imx6solosabreauto
 - imx6solosabresd

Change-Id: I37e9c3737552299677e315b6279a6e0a8e217836
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>