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/file/Makefile.am.include b/file/Makefile.am.include
new file mode 100644
index 0000000..155db50
--- /dev/null
+++ b/file/Makefile.am.include
@@ -0,0 +1 @@
+mboxd_SOURCES += %reldir%/backend.c