[Mesh] Hardening & build tweaks

Here are my tweaks to the firmware from a few days back. Changes made: * build script uses /bin/bash instead of /bin/sh * Patches the openwrt telnetd to accept a -I interfacename option (in openwrt_addons/package/busybox/patches/960-telnetd-bindtodevice.diff) * refactored check to see if root PW is set into files/lib/functions/auth.sh * files/etc/init.d/telnet now sets the -I eth0 option, and will set the IP default TTL to 1 if the root PW isn't set. * files/etc/hotplug.d/iface/20-ntpclient will only be run if the root PW has been set. * prepare - copies openwrt_addons into built_firmware/openwrt/ before building; now just copies openwrt_config/feeds.conf into the build dir, moving the old one to feeds.conf.old if they differed. This fixes the bug where re-running prepare would cause piles of spurious errors as feeds.conf grew.