Add ADB protocol files from blargg's work

- <http://geekhack.org/index.php?topic=14290.msg1070604#msg1070604>

converter/adb_usb/adb_blargg.h

@@ -0,0 +1,38 @@
+// Basic support for ADB keyboard
+
+#ifndef ADB_BLARGG_H
+#define ADB_BLARGG_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+// Sets up ADB bus. Doesn't send anything to keyboard.
+void adb_host_init( void );
+
+// Receives key press event from keyboard.
+// 0xKKFF: one key changed state
+// 0xKKKK: two keys changed state
+enum { adb_host_nothing = 0 }; // no keys changed state
+enum { adb_host_error = 0xFFFE }; // receive error
+uint16_t adb_host_kbd_recv( void );
+
+// Current state of keyboard modifiers and a few other keys
+// Returns adb_host_nothing if keyboard didn't respond.
+// Returns adb_host_error if error receiving.
+uint16_t adb_host_kbd_modifiers( void );
+
+// Sends command and two bytes of data to keyboard
+void adb_host_listen( uint8_t cmd, uint8_t data_h, uint8_t data_l );
+
+// Sets keyboard LEDs. Note that bits are inverted here, so 1 means off, 0 means on.
+void adb_host_kbd_led( uint8_t led );
+
+// State of power switch (false = pressed), or true if unsupported
+bool adb_host_psw( void );
+
+
+// Legacy support
+#define ADB_POWER       0x7F
+#define ADB_CAPS        0x39
+
+#endif