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

[Troy Kisky]

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