Standalone USB Joystick with bootlader

This pull replaces #696
This pull fixes  #545

Functionality:
* USB HID Joystick only mode in openTX 
* USB Mass storage mode removed from openTX (this is now supported by Bootloader)

Here is a short description of new functionality:


### Description of Joystick interface
Taranis simulates a joystick device with:
* 8 analog axes
* 8 digital buttons

Analog axes are mapped to first eight channels:  from CH1 to CH8. Valid output value for each channel is from -100 to 100 and is mapped into 8 bit signed number for -127 to 127. Higher or lower channel values are truncated into -100 or +100 respectively. 

Buttons are mapped to next eight channels: form CH9 to CH16. Button is considered pressed when channel value is bigger than zero.

User should create a new model for joystick usage with:
* TX module turned off
* servo limits at 100%
* appropriate mixes on channels 1 to 16
* optionally some exponential on analog axes (channels 1 - 8)