package/erlang: export EI_VSN so other packages can use it

Packages that depend on Erlang may need the Erlang Interface Version
(aka EI_VSN), so just export it.

We do export a static value instead of looking in the file, to avoid
spawning a shell every time the variable is dereferenced.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[johan.oudinet@gmail.com: update EI_VSN since the erlang update]
Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>