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/starfield.c

@@ -19,10 +19,11 @@
 
 #include "plugin.h"
 #include "lib/helper.h"
+#include "lib/pluginlib_exit.h"
 
 #ifdef HAVE_LCD_BITMAP /* and also not for the Player */
 
-PLUGIN_HEADER
+
 
 /******************************* Globals ***********************************/
 
@@ -497,9 +498,11 @@ int plugin_main(void)
             case STARFIELD_RC_QUIT:
 #endif
             case(STARFIELD_QUIT):
-            case(SYS_USB_CONNECTED):
                 return PLUGIN_OK;
                 break;
+            default:
+                exit_on_usb(button);
+                break;
         }
     }
 }