FEATURE: add recovery option using file checking on SD card

[davidpelfree]

Hi.

I want to suggest a simple idea I saw here:
https://www.davidgouveia.net/2016/03/avoid-being-locked-out-of-your-zsun-wifi-reader/

The idea is to reset the device settings if a "/restore" file exists in SD card.
It's implemented using a simple init.d script.

This idea is much simpler than current method: trying to insert/remove SD card during boot to simulate a button click.

I tried this idea myself and it works nice. However, after resetting the device settings, the script itself is also deleted... This means that if I forgot to create it again, the next time, I will be stuck with inaccessible device (and have to use the less-convenient current method of SD card insertion during boot).

Adding this init.d script into the openwrt-zsun firmware code will make it much easier to ALWAYS reset the device in case of misconfiguration, without worrying if the script exists or not. It will always be there.