Example build invocations:

mihawk:
MACHINE=mihawk bitbake obmc-phosphor-image
p10bmc:
MACHINE=p10bmc bitbake obmc-phosphor-image
swift:
MACHINE=swift bitbake obmc-phosphor-image
witherspoon:
MACHINE=witherspoon bitbake obmc-phosphor-image
witherspoon-tacoma:
MACHINE=witherspoon-tacoma bitbake obmc-phosphor-image