Add support for hidg devices on Linux

There is a HID gadget driver on Linux which provides emulation of USB
HID devices. This could be very useful for testing the Solo firmware
without actual hardware, using only a Linux box.

This patch adds a command line argument which specifies whether the
existing UDP backing should be used or the new one which reads and
writes to /dev/hidg0.

Testing done:
 1. Created HID device with configfs
 2. Started "./main -b hidg" as root
 3. Successfully executed Webauthn registration and authentication on
 the same Linux machine

Closes: #122

pc/app.h

@@ -7,6 +7,7 @@
 
 #ifndef SRC_APP_H_
 #define SRC_APP_H_
+#include <stdbool.h>
 
 #define USING_DEV_BOARD
 
@@ -20,6 +21,8 @@
 
 void printing_init();
 
+extern bool use_udp;
+
 //                              0xRRGGBB
 #define LED_INIT_VALUE			0x000800
 #define LED_WINK_VALUE			0x000008