Please add this hint about non-destructive testing of the U-Boot binary

Hi,

after having read a lot about how to make my TL-WDR3600 behave (read: OpenWrt + extroot + Debian planned), I figured that it would be possible to execute-test the u-boot binary prior to actually writing it, thus I added these changes.
Unfortunately this step is UNTESTED since I don't even have working serial comm soldered yet
(side snide remark: somehow updating an ASUS WL-500gP v2 is *MUCH* simpler since a bootup-time rescue button press will already cause the device's bootloader to listen for an incoming TFTP upload on its own. Not really happy here on WDR3600, where a device using a custom-bent kernel firmware ending up semi-bricked seems to be a lot more problematic)

Feel free to liberally apply my contributions, with or without precise attribution/commits, or liberally reworded (e.g. if you managed to test my step).

Thanks a TON for your very cool U-Boot work!