Update version for v3.0.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 13565e6..65be8fd 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.12.90
+2.12.91