Update BINDIR paths in case scripts aren't called with matching path and working directories.
Allow SUDO and PLUGINDIR to be over-written in the environment when
calling scripts.  This fixes non-pi install issues for plugins.
Don't use cwd directory for the plugin location, apparently somehow this
can return "/" in some instances.  Probably when init kicks rcS which
kicks rc.local which is where we start.  I couldn't reproduce the
problem, but let's just set a default of /opt/fpp/plugins and allow the
user to override/configure this setting if they have a non-standard
installation.