why conditional assignment of DEPLOY_DIR_IMAGE in bitbake.conf?

["Robert P. J. Day" <rpjday@crashcourse.ca>]

  quite possibly another silly question, but i'm perusing poky's
version of bitbake.conf and i see this:

DEPLOY_DIR ?= "${TMPDIR}/deploy"
DEPLOY_DIR_TAR = "${DEPLOY_DIR}/tar"
DEPLOY_DIR_IPK = "${DEPLOY_DIR}/ipk"
DEPLOY_DIR_RPM = "${DEPLOY_DIR}/rpm"
DEPLOY_DIR_DEB = "${DEPLOY_DIR}/deb"
DEPLOY_DIR_IMAGE ?= "${DEPLOY_DIR}/images/${MACHINE}"
DEPLOY_DIR_TOOLS = "${DEPLOY_DIR}/tools"

  now, what is the value of using "?=" to set DEPLOY_DIR_IMAGE, rather
than just "=". i know what "?=" represents, but i normally expect to
see it in a context where someone might have set it earlier to some
other value, but this is in bitbake.conf, before any of the "include"
or "require" directives to pull in any of the other .conf files. so in
the midst of all those other DEPLOY_DIR_* hard assignments, why is the
images directory a conditional install?

rday

-- 

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Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
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