Example build invocations:

mihawk:
DISTRO=openbmc-witherspoon MACHINE=mihawk bitbake obmc-phosphor-image
p10bmc:
DISTRO=openbmc-openpower MACHINE=p10bmc 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 obmc-phosphor-image