erlang-rebar: fix license variables

The erlang-rebar package uses ERLANG_ as the prefix for its license
variables, therefore overriding the license information from the
erlang package. This commit fixes this by using the appropriate
ERLANG_REBAR_ suffix.

Reported-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>