[meta-freescale] what means 'PREFERRED_PROVIDER_linux-imx ??= "4.1.15"' ??

[rpjday at crashcourse.ca]

   in current meta-freescale layer, conf/machine/imx6sllevk.conf,
one reads:

   PREFERRED_PROVIDER_linux-imx ??= "4.1.15"

should that not use "PREFERRED_VERSION? or is this some strange
variation of that variable of which i am unaware?

rday