buildtools-tarball: export OPENSSL_CONF for openssl

export OPENSSL_CONF to aviod SDK openssl can not find openssl.cnf.

(From OE-Core rev: 0aaf3dd1)

(From OE-Core rev: 63d8569b)

(From OE-Core rev: e733a5f3)

Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>