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>

package/erlang/erlang.mk

@@ -4,6 +4,7 @@
 #
 ################################################################################
 
+# See note below when updating Erlang
 ERLANG_VERSION = 17.4
 ERLANG_SITE = http://www.erlang.org/download
 ERLANG_SOURCE = otp_src_$(ERLANG_VERSION).tar.gz
@@ -16,6 +17,10 @@ ERLANG_INSTALL_STAGING = YES
 # Touching erts/configure.in
 ERLANG_AUTORECONF = YES
 
+# Whenever updating Erlang, this value should be updated as well, to the
+# value of EI_VSN in the file lib/erl_interface/vsn.mk
+ERLANG_EI_VSN = 3.7.20
+
 # The configure checks for these functions fail incorrectly
 ERLANG_CONF_ENV = ac_cv_func_isnan=yes ac_cv_func_isinf=yes