gluon-core: util: check if file exists prior to reading

The file_contains_line helper function was not testing whether a file
exists or not prior attempting to read from it.

Add this check to circumvent errors on the private WiFi config in
case the hwflags file is missing.

Reported-by: Tom Herbers <freifunk@tomherbers.de>
Tested-by: Tom Herbers <freifunk@tomherbers.de>
Signed-off-by: David Bauer <mail@david-bauer.net>