bitbake: bitbake-prserver: use PRSERV_UPSTREAM as default setting

Instead of PRSERVER_UPSTREAM.
The intended variable name is PRSERV_UPSTREAM, as
already used in lib/prserv/serv.py, an consistently
with the PRSERV_HOST variable name.

(Bitbake rev: b0c277f16f9fae51914024c1daecd5d3e4fac5c2)

Signed-off-by: Michael Opdenacker <michael.opdenacker@rootcommit.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake-prserv

@@ -80,7 +80,7 @@ def main():
     parser.add_argument(
         "-u",
         "--upstream",
-        default=os.environ.get("PRSERVER_UPSTREAM", None),
+        default=os.environ.get("PRSERV_UPSTREAM", None),
         help="Upstream PR service (host:port)",
     )