ubb-vga2: added support for showing a PPM image

- Makefile (OBJS): added ppm.o and ppmimg.o
- ppm.h (load_ppm), ppm.c: PPM file loader, adapted from
  eda-tools/schhist/ppmdiff/ppmdiff.c
- ubb-vga.h (img_name, ppmimg), ppmimg.c: PPM image to frame buffer
  converter
- ubb-vga2.c (usage, main): the threshold is now set with the option -l
- ubb-vga2.c (usage, main): if a second argument is given, treat it as
  a PPM file
- ubb-vga2.c (usage): also documented option -t

ubb-vga/ppm.h

@@ -0,0 +1,23 @@
+/*
+ * ppm.h - Load a PPM image
+ *
+ * Written 2011 by Werner Almesberger
+ * Copyright 2011 Werner Almesberger
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ */
+
+
+#ifndef	PPM_H
+#define	PPM_H
+
+#include <stdint.h>
+
+
+uint8_t *load_ppm(const char *name, int *x, int *y);
+
+#endif /* !PPM_H */
+