Example build invocations:

pstbmc:
MACHINE=pstbmc bitbake obmc-phosphor-image
p10bmc:
MACHINE=p10bmc bitbake obmc-phosphor-image
witherspoon:
MACHINE=witherspoon bitbake obmc-phosphor-image
witherspoon-tacoma:
MACHINE=witherspoon-tacoma bitbake obmc-phosphor-image