[kirkstone] Update support for official meta-freescale compatible with kirkstone
Made IMX_DEFAULT_KERNEL variable compatible with newest Yocto's syntax, replacing "_" with ":" for particular case of "mx6" and "mx8" MACHINEOVERRIDE
Made IMX_DEFAULT_KERNEL variable compatible with newest Yocto's syntax, replacing "_" with ":" for particular case of "mx6" and "mx8" MACHINEOVERRIDE