mboxd: Add backend DBus interface and commandline options

Also implement a backend commandline option to mboxctl: `mboxctl
--backend ...`, to allow easy run-time switching of the backend from the
commandline.

Switching between VPNOR and file backends via mboxctl was tested on
Witherspoon, and MTD and file backends on Romulus.

Change-Id: Iaf0e27ecf1d5cdd9e3a31729fb179096bbc37408
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mboxd.h b/mboxd.h
index 7930242..ef4f5be 100644
--- a/mboxd.h
+++ b/mboxd.h
@@ -76,7 +76,7 @@
 	struct backend backend;
 
 	/* Commandline parameters */
-	const char *path;
+	const char *source;
 
 /* System State */
 	enum mbox_state state;