m53evk: Remove duplicated setting of U-Boot preferred provider

The system already default for u-boot-fslc as provider so we shouldn't
hardcode it here.

Change-Id: I4472561b74f857813b5f1f2b9d8ed2d68f479c8d
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
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:

 - imx233-olinuxino-*
 - m28evk
 - m53evk

Change-Id: I824e818f50de094fe1b25ef3ea202abd90093650
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>