sanitycheck: use Unknown instead of N/A for fail reason

Instead of N/A, use Unknown which is more appropriate, N/A does not
really apply here.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

scripts/sanitycheck

diff --git a/scripts/sanitycheck b/scripts/sanitycheck
index 518e0de..f3edb90 100755
--- a/scripts/sanitycheck
+++ b/scripts/sanitycheck
@@ -1645,7 +1645,7 @@
         self.platform = platform
 
         self.status = None
-        self.reason = "N/A"
+        self.reason = "Unknown"
         self.metrics = dict()
         self.handler = None
         self.outdir = outdir