[U-Boot] Single u-boot for quad/duallite

Since we are slowly heading toward a single u-boot for quad/duallite, can we find a solution to this now.

mx6sabre_common.h: "fdt_file=" CONFIG_DEFAULT_FDT_FILE "\0" \ mx6sabresd.h:#define CONFIG_DEFAULT_FDT_FILE "imx6q-sabresd.dtb" mx6sabresd.h:#define CONFIG_DEFAULT_FDT_FILE "imx6dl-sabresd.dtb"

Perhaps common code could create this environment variable by concatenating with the board name if not already existing in the environment.

Opinions?

Thanks Troy