mboxd: Add support for file-backed flash devices.

This commit adds a new file based backing source.  The file based
backing source takes a raw pnor image that is usually flashed to a mtd
device.

This backing source enabled rapid testing of pnor images.

Tested on Witherspoon with the VPNOR and file backends, and Romulus for
the MTD and file backends.

Change-Id: I253ecfa6b58d071c7982f153ad50da8e4ad39fa2
Signed-off-by: Evan Lojewski <github@meklort.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/Makefile.am b/Makefile.am
index 0ce5242..27d63a4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,6 +19,9 @@
 # MTD Backing storage
 include mtd/Makefile.am.include
 
+# File Backing storage
+include file/Makefile.am.include
+
 if VIRTUAL_PNOR_ENABLED
 # VPNOR Backing storage
 include vpnor/Makefile.am.include