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/ubb-vga.h

@@ -26,10 +26,20 @@
 void *map(off_t addr, size_t size);
 
 
+/* grabfb.c */
+
 extern uint8_t thres;
 
 void grabfb(void *f, int xres, int yres);
 
+/* tstimg.c */
+
 void tstimg(void *f, int xres, int yres);
 
+/* ppmimg.c */
+
+extern char *img_name;
+
+void ppmimg(void *f, int xres, int yres);
+
 #endif /* !UBB_VGA_H */