`fg` stderr output leaks to user logs

[dfunckt]

When the user's app exits fg complains that the process terminated:

/usr/bin/entry.sh: line 93: fg: job has terminated

This output is captured by the Supervisor and sent over to the Dashboard as if it was output from the user application.

This is due to fg's stderr not being redirected to /dev/null here: https://github.com/resin-io-library/resin-rpi-raspbian/blob/master/entry.sh#L93

That is without using systemd.

[nghiant2710]

fixed in #63