release: Zephyr 2.6.0-rc1

Set version to 2.6.0-rc1

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
diff --git a/VERSION b/VERSION
index 5faa288..974b252 100644
--- a/VERSION
+++ b/VERSION
@@ -1,5 +1,5 @@
 VERSION_MAJOR = 2
-VERSION_MINOR = 5
-PATCHLEVEL = 99
+VERSION_MINOR = 6
+PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION =
+EXTRAVERSION = rc1