erlang-relx: pull in upstream fix and simplify

The upstream fix addressed the need to build in a directory
called relx instead of any directory name. Also, make this a
host-only package since it doesn't make sense on target.
erlang-rebar: make host-only

This is another tool that doesn't make much sense on
the target.
nerves-demo: compile relx and rebar
Remove previous attampt at pulling in Erlang apps

The new approach is to just create releases rather than integrate
Erlang apps into buildroot.