witherspoon-128: Use ${MACHINE} for device tree file name

Replace "witherspoon-128" with ${MACHINE} for the device tree file name
as it's better practice to use bitbake variables instead of hard-coding
values.

Tested:
    Output of "bitbake obmc-phosphor-image -e":
    # pre-expansion value:
    #   "${KMACHINE}-bmc-opp-${MACHINE}.dtb"
    KERNEL_DEVICETREE="aspeed-bmc-opp-witherspoon-128.dtb"

Change-Id: I568ac2fe3705bd27331d8631e47a4e49c3f97f41
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
1 file changed