release: Zephyr 2.3.0-rc2

Set version to 2.3.0-rc2.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/VERSION b/VERSION
index 7f34cd1..42f41a8 100644
--- a/VERSION
+++ b/VERSION
@@ -2,4 +2,4 @@
 VERSION_MINOR = 3
 PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION = rc1
+EXTRAVERSION = rc2