Update version for v7.0.0-rc0 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

VERSION

diff --git a/VERSION b/VERSION
index 8e8e37d..064ddf9 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-6.2.50
+6.2.90