openpower: Add msglog tool to skiroot

This adds a one line shell script that can be used to display the OPAL
msglog without having to type out the full path.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/overlay/bin/msglog b/openpower/overlay/bin/msglog
new file mode 100755
index 0000000..6ee64b9
--- /dev/null
+++ b/openpower/overlay/bin/msglog
@@ -0,0 +1,2 @@
+#!/bin/sh
+exec cat /sys/firmware/opal/msglog