rules.h: Use more consistent naming

Use 'ENV' consistently and drop the redundant 'STAGE' in the naming.

Change-Id: I51f2a7e70eefad12aa214e92f23e5fd2edf46698
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64296
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/include/thread.h

@@ -37,7 +37,7 @@ int thread_run_until(struct thread_handle *handle, enum cb_err (*func)(void *),
 /* Waits until the thread has terminated and returns the error code */
 enum cb_err thread_join(struct thread_handle *handle);
 
-#if ENV_STAGE_SUPPORTS_COOP
+#if ENV_SUPPORTS_COOP
 
 struct thread {
 	int id;