style(expect): normalize debugging messages

A minor style cleanup to the messages printed by the Expect scripts to
make them a bit more consistent and descriptive.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: Ibd8ccb4423bd5aaeffc99c2f3602123794dc6200
(cherry picked from commit 78ea2c38582da41eb8f9509a2130e702cdf0f1c9)
diff --git a/expect/terminable.inc b/expect/terminable.inc
index a96d51a..7c8ca17 100644
--- a/expect/terminable.inc
+++ b/expect/terminable.inc
@@ -5,6 +5,6 @@
 #
 
 trap {
-    puts "<<received SIGINT, exiting>>"
+    message "received SIGINT, exiting"
     exit_uart 0
 } SIGINT