Don't enable thread preemption during S3 resume vga option rom execution.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/stacks.c b/src/stacks.c
index 6bcb319..beccc0f 100644
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -287,7 +287,7 @@
 int
 threads_during_optionroms(void)
 {
-    return CONFIG_THREADS && ThreadControl == 2;
+    return CONFIG_THREADS && ThreadControl == 2 && in_post();
 }
 
 // Switch to next thread stack.