The document to descript change the u-boot environment variables under the Linux rootfs.
Also provide a demo on i.MX6ull evk of sdcard mirror.
Linux fw_printenv fw_setenv to access U-Boot's environment variables.pdf --- the document
fw_printenv_fw_setenv_demo_iMX6ullevk_L4.14.98_2.0.0_ga.sdcard --- demo sdcard mirror
