openssl: export necessary env vars in SDK

In current SDK, when running the following command in python
shell, we get an error.

$ python3
>>> from cryptography.hazmat.backends import openssl

The error message is as below:

  cryptography.exceptions.InternalError: Unknown OpenSSL error.

We could set OPENSSL_MODULES explicitly in nativesdk-openssl package
so that when SDK is set up, it's in environment and we can
get rid of the above error.

Also, there are other env vars that need to be exported. And we export
all of them to keep sync with openssl-native.bbclass.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>