Sample meta-witherspoon build invocations:

mihawk:
DISTRO=openbmc-witherspoon MACHINE=mihawk bitbake obmc-phosphor-image
swift:
DISTRO=openbmc-witherspoon MACHINE=swift bitbake obmc-phosphor-image
witherspoon:
DISTRO=openbmc-witherspoon MACHINE=witherspoon bitbake obmc-phosphor-image
witherspoon-tacoma:
DISTRO=openbmc-openpower MACHINE=witherspoon-tacoma bitbake aspeed-image-norootfs
witherspoon-128:
DISTRO=openbmc-witherspoon MACHINE=witherspoon-128 bitbake obmc-phosphor-image