Use fwrite instead of exec to ensure single quotes can be handled in PSK

[lachiemurray]

#6

Variable interpolation was causing the exec argument to expand to something like:

echo 'ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev\nupdate_config=1\ncountry=GB\n\n\nnetwork={\n\tssid="MySSID"\n\t#psk="MyPskWithSingleQuote' "\n\tpsk=psketc\n}\n' > /tmp/wifidata

This results in the following error:

sh: 11: Syntax error: Unterminated quoted string

Caused by the extra single quote in the middle of the PSK:

...#psk="MyPskWithSingleQuote' "\n\tp...

Proposed solution is to use fwrite instead of exec.

Tested on my emonpi by connecting in AP mode and then connecting to home WiFi network

[lachiemurray]

Just flagging this again @emrysr @glynhudson

[TrystanLea]

Sorry @lachiemurray to miss your pull request here, I will test and hopefully merge shortly!