Updated instructions for firmware updates using tftp and scp

[cdg123]

As a relatively new user to updating the software build on a camera manually using sysupgrade I found the instructions a little confusing and so have made some changes so hopefully more novice users can avoid any issues or time wasted troubleshooting issues such as with getting scp to work.

I have tested the tftp and scp instructions with a new hi3516ev300 board with both prebuilt images from Github and my own build locally but have not got an sdcard slot to prove the instructions but as i have not changed them from the original assume it is all ok.

Please update or change anything you feel necessary to fit with the wiki design/template or naming