common: Move get_dev_mtd() to mtd.c

Allows tests to link in alternative implementations of get_dev_mtd()
without losing other functions available in common.c.

Change-Id: I606a83aa9bc59b5fbab1fb11ee1e2574c7f6d127
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/Makefile.am b/Makefile.am
index c5b821f..4dbab78 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,7 +1,7 @@
 ACLOCAL_AMFLAGS = -I m4
 sbin_PROGRAMS = mboxd mboxctl
 
-mboxd_SOURCES = mboxd.c common.c mboxd_dbus.c mboxd_flash.c mboxd_lpc.c mboxd_msg.c mboxd_windows.c
+mboxd_SOURCES = mboxd.c common.c mboxd_dbus.c mboxd_flash.c mboxd_lpc.c mboxd_msg.c mboxd_windows.c mtd.c
 mboxd_LDFLAGS = $(LIBSYSTEMD_LIBS)
 mboxd_CFLAGS = $(LIBSYSTEMD_CFLAGS)