Update version for v2.6.0-rc0 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index f6bb220..b042f94 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.50
+2.5.90