Update version for v2.0.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 32c79ec..0ff4310 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.7.90
+1.7.91