openssl: Fix SDK environment script to avoid unbound variable

Avoid errors like:

buildtools/sysroots/x86_64-pokysdk-linux/environment-setup.d/openssl.sh: line 6: BB_ENV_PASSTHROUGH_ADDITIONS: unbound variable

by setting an explicit empty default value.

(From OE-Core rev: a57192131cbcb65e17b11f47aa0f90ef63258280)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5a2a4910a22668f25679a47deaa9e2ed28665efa)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-connectivity/openssl/files/environment.d-openssl.sh

@@ -3,4 +3,4 @@ export SSL_CERT_DIR="$OECORE_NATIVE_SYSROOT/usr/lib/ssl/certs"
 export SSL_CERT_FILE="$OECORE_NATIVE_SYSROOT/usr/lib/ssl/certs/ca-certificates.crt"
 export OPENSSL_MODULES="$OECORE_NATIVE_SYSROOT/usr/lib/ossl-modules/"
 export OPENSSL_ENGINES="$OECORE_NATIVE_SYSROOT/usr/lib/engines-3"
-export BB_ENV_PASSTHROUGH_ADDITIONS="$BB_ENV_PASSTHROUGH_ADDITIONS SSL_CERT_DIR SSL_CERT_FILE OPENSSL_CONF OPENSSL_MODULES OPENSSL_ENGINES"
+export BB_ENV_PASSTHROUGH_ADDITIONS="${BB_ENV_PASSTHROUGH_ADDITIONS:-} SSL_CERT_DIR SSL_CERT_FILE OPENSSL_CONF OPENSSL_MODULES OPENSSL_ENGINES"