firmware: More common infrastructure for USB handling

Let's move low-level handling of endpoint queue refill from the
individual apps into common/shared code.

Now the main simply has to call usb_process() for every interface,
and inbound messages will be dispatched to call-back functions from
there.

Change-Id: Ic6f9c6c1ffdbb0c9c3b284371ecc83b17e3be746

firmware/apps/cardem/main.c

@@ -221,6 +221,7 @@ extern int main(void)
 			}
 			last_simtrace_config = simtrace_config;
 		} else {
+			//FIXME: usb_proces() for every interface in this configuration?
 			if (config_func_ptrs[simtrace_config].run) {
 				config_func_ptrs[simtrace_config].run();
 			}