In old Freescale Linux release, you can easily found kernel low-level debugging functions options when you run command 'make menuconfig'. But in new release version, you can not see it directly. You can easily to enable this option by the following steps.
$ cd kernel_imx
$ make menuconfig
kernel hacking ----> [*] kernel debugging /* please select this option first, then you can find the next option */
----> [*] kernel low-level debugging functions