Second try: Introduce plugin_crt0.c that every plugin links.

It handles exit() properly, calling the handler also when the plugin returns
normally (also make exit() more standard compliant while at it).
It also holds PLUGIN_HEADER, so that it doesn't need to be in each plugin anymore.

To work better together with callbacks passed to rb->default_event_handler_ex() introduce exit_on_usb() which will call the exit handler before showing the usb screen and exit() after it.
In most cases rb->default_event_handler_ex() was passed a callback which was manually called at all other return points. This can now be done via atexit().

In future plugin_crt0.c could also handle clearing bss, initializing iram and more.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27873 a1c6a512-1295-4272-9138-f99709370657

apps/plugins/test_greylib_bitmap_scale.c

@@ -44,7 +44,7 @@
 #define BMP_LOAD rb->read_bmp_file
 #endif
 
-PLUGIN_HEADER
+
 GREY_INFO_STRUCT
 static unsigned char grey_bm_buf[LCD_WIDTH * LCD_HEIGHT + 
                    BM_SCALED_SIZE(LCD_WIDTH,0,FORMAT_NATIVE,0)];