Adding dynamic USB power support

- Each scan module now has a current change callback which passes the available current as a parameter
- No longer attempts to use the max 500 mA immediately, starts with 100 mA then goes to 500 mA after enumeration
- If enumeration fails due to bMaxPower of 500 mA, then attempt again at 100 mA (might also be possible to go even lower to 20 mA in certain cases)
- Now working with the Apple Ipad (no over-power messages)
- Fixed Wake-up behaviour on Apple Ipad (and likely other iOS devices)
- More effecient set_feature/clear_feature handling (device handler)
- Initial power handling via Interconnect (still needs work to get it more dynamic)

Output/pjrcUSB/arm/usb_desc.c

@@ -1,7 +1,7 @@
 /* Teensyduino Core Library
  * http://www.pjrc.com/teensy/
  * Copyright (c) 2013 PJRC.COM, LLC.
- * Modified by Jacob Alexander (2013-2015)
+ * Modified by Jacob Alexander (2013-2016)
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
@@ -451,7 +451,7 @@ static uint8_t config_descriptor[CONFIG_DESC_SIZE] = {
 	1,                                      // bConfigurationValue
 	0,                                      // iConfiguration
 	0xA0,                                   // bmAttributes
-	250,                                    // bMaxPower
+	250,                                    // bMaxPower - Entry Index 8
 
 // --- Keyboard HID --- Boot Mode Keyboard Interface
 // - 9 bytes -
@@ -695,6 +695,8 @@ static uint8_t config_descriptor[CONFIG_DESC_SIZE] = {
 	SYS_CTRL_INTERVAL,                      // bInterval
 };
 
+uint8_t *usb_bMaxPower = &config_descriptor[8];
+
 
 
 // ----- String Descriptors -----