Linux fw_printenv fw_setenv to access U-Boot's environment variables

[#1]

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