[makefile] Add -Wformat-nonliteral as an extra warning category

-Wformat-nonliteral is not enabled by -Wall and needs to be explicitly
 specified.

Modified the few files that use nonliteral format strings to work with
this new setting in place.

Inspired by a patch from Carl Karsten <carl@personnelware.com> and an
identical patch from Rorschach <r0rschach@lavabit.com>.

src/Makefile.housekeeping

 #
 CFLAGS		+= -I include -I arch/$(ARCH)/include -I .
 CFLAGS		+= -Os -ffreestanding
-CFLAGS		+= -Wall -W
+CFLAGS		+= -Wall -W -Wformat-nonliteral
 CFLAGS		+= -g
 CFLAGS		+= $(EXTRA_CFLAGS)
 ASFLAGS		+= $(EXTRA_ASFLAGS)

src/drivers/net/3c595.c

         if (n > 0) {
           printf("/");
 	}
-        printf(conn_tab[k].name);
+        printf("%s", conn_tab[k].name );
         n++;
       }
     }

src/drivers/net/forcedeth.c

 		delaymax -= delay;
 		if (delaymax < 0) {
 			if (msg)
-				printf(msg);
+				printf("%s", msg);
 			return 1;
 		}
 	} while ((readl(base + offset) & mask) != target);

src/drivers/net/via-rhine.c

     unsigned char mode3_reg;
 
     if (rhine_debug > 0 && did_version++ == 0)
-	printf (version);
+	printf ("%s",version);
 
     // get revision id.
     pci_read_config_byte(pci, PCI_REVISION, &revision_id);

src/include/gpxe/xfer.h

 			      const void *data, size_t len );
 extern int xfer_vprintf ( struct xfer_interface *xfer,
 			  const char *format, va_list args );
-extern int xfer_printf ( struct xfer_interface *xfer,
-			 const char *format, ... );
+extern int __attribute__ (( format ( printf, 2, 3 ) ))
+xfer_printf ( struct xfer_interface *xfer, const char *format, ... );
 extern int xfer_seek ( struct xfer_interface *xfer, off_t offset, int whence );
 
 extern void ignore_xfer_close ( struct xfer_interface *xfer, int rc );

src/net/tcp/ftp.c

  *
  */
 
+/** An FTP control channel string */
+struct ftp_control_string {
+	/** Literal portion */
+	const char *literal;
+	/** Variable portion
+	 *
+	 * @v ftp	FTP request
+	 * @ret string	Variable portion of string
+	 */
+	const char * ( *variable ) ( struct ftp_request *ftp );
+};
+
 /**
- * FTP control channel strings
+ * Retrieve FTP pathname
  *
- * These are used as printf() format strings.  Since only one of them
- * (RETR) takes an argument, we always supply that argument to the
- * snprintf() call.
+ * @v ftp		FTP request
+ * @ret path		FTP pathname
  */
-static const char * ftp_strings[] = {
-	[FTP_CONNECT]	= NULL,
-	[FTP_USER]	= "USER anonymous\r\n",
-	[FTP_PASS]	= "PASS etherboot@etherboot.org\r\n",
-	[FTP_TYPE]	= "TYPE I\r\n",
-	[FTP_PASV]	= "PASV\r\n",
-	[FTP_RETR]	= "RETR %s\r\n",
-	[FTP_WAIT]	= NULL,
-	[FTP_QUIT]	= "QUIT\r\n",
-	[FTP_DONE]	= NULL,
+static const char * ftp_uri_path ( struct ftp_request *ftp ) {
+	return ftp->uri->path;
+}
+
+/** FTP control channel strings */
+static struct ftp_control_string ftp_strings[] = {
+	[FTP_CONNECT]	= { NULL, NULL },
+	[FTP_USER]	= { "USER anonymous", NULL },
+	[FTP_PASS]	= { "PASS etherboot@etherboot.org", NULL },
+	[FTP_TYPE]	= { "TYPE I", NULL },
+	[FTP_PASV]	= { "PASV", NULL },
+	[FTP_RETR]	= { "RETR ", ftp_uri_path },
+	[FTP_WAIT]	= { NULL, NULL },
+	[FTP_QUIT]	= { "QUIT", NULL },
+	[FTP_DONE]	= { NULL, NULL },
 };
 
 /**
  *
  */
 static void ftp_next_state ( struct ftp_request *ftp ) {
+	struct ftp_control_string *ftp_string;
+	const char *literal;
+	const char *variable;
 
 	/* Move to next state */
 	if ( ftp->state < FTP_DONE )
 		ftp->state++;
 
 	/* Send control string if needed */
-	if ( ftp_strings[ftp->state] != NULL ) {
-		DBGC ( ftp, "FTP %p sending ", ftp );
-		DBGC ( ftp, ftp_strings[ftp->state], ftp->uri->path );
-		xfer_printf ( &ftp->control, ftp_strings[ftp->state],
-			      ftp->uri->path );
-	}	
+	ftp_string = &ftp_strings[ftp->state];
+	literal = ftp_string->literal;
+	variable = ( ftp_string->variable ?
+		     ftp_string->variable ( ftp ) : "" );
+	if ( literal ) {
+		DBGC ( ftp, "FTP %p sending %s%s\n", ftp, literal, variable );
+		xfer_printf ( &ftp->control, "%s%s\r\n", literal, variable );
+	}
 }
 
 /**