release: Zephyr 2.3.0-rc1

Set version to 2.3.0-rc1.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/VERSION b/VERSION
index c966799..7f34cd1 100644
--- a/VERSION
+++ b/VERSION
@@ -1,5 +1,5 @@
 VERSION_MAJOR = 2
-VERSION_MINOR = 2
-PATCHLEVEL = 99
+VERSION_MINOR = 3
+PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION =
+EXTRAVERSION = rc1