erlang: set ERL_TOP to avoid using user's version

Explicitly set the ERL_TOP environment variable to override values set
on a user's system. This prevents Erlang's configure scripts from
using source files outside of the ones that Buildroot downloaded.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>