Update version for v3.0.0-rc0 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 1195214..13565e6 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.12.50
+2.12.90