Updates dhcp-all-interfaces so that we avoid using
ifquery on Fedora to determine if an interface exists.
Adds a new config_exists function which simply looks for
the ifcfg config file on distro's that use netscripts.
Change-Id: Ie55524b83820fe4fab28cc9d3f7e08ca2e42c182
Rename generate-interfaces-file.sh to dhcp-all-interfaces.sh
so that it is easier to find/remember in the $PATH on a deployed
image.
Also, on some distros the script actually generates many files
so the previous name was a bit confusing.
Change-Id: I7152fa7c28e8ade251311da2cd5f75972423b66c