USB-Core Fixes

[NicoHood]

Backported a few fixes from https://github.com/NicoHood/HID

• Flexible, global Endpoint size (USB_EP_SIZE)
• Added Weak CDC LineEncoding functions
• Added CDC LineEncoding information functions
• Fixed Magic Key for newer boards
• Added some useful definitions
• Minor fixes
• Added u2 Series support

Keywords for the Serial functions should be added as well. Inlining those functions doesnt work since its a static variable. I think this is the most save and easy way to access the data for advanced users.

Should we change the Bootkey (for the non 32u4 boards) to a 1byte value? Thats normally enough and saves flash in the bootloader and sketch itself. I'd go for it, since I have to change my HoodLoader2 anyways.

[matthijskooijman]

IMHO, a single big commit that just says "Applied fixes from HID-Project development" messes up the history badly, those changes should at least be documented in detail, but preferably be split into multiple commits. If you already have them as separate commits in your HID-project repository, you can probably fiddle a bit with git format-patch and git am to port over the separate commits.

Regarding the baud(), dtr(), etc. accessors, I also added these in #3343, so it might be better to drop those. Regarding the weak event callbacks, also see the discussion in #3343 where some objections to such a weak API are made. What is the actual usecase you need these week APIs for?

[NicoHood]

I am not that git pro. But I can make a new PR if wanted.

The Events were made for this usage:
https://github.com/NicoHood/HID/blob/master/examples/Projects/USB-Serial/USB-Serial.ino

[matthijskooijman]

@NicoHood You should be able to implement the same sketch by simply polling the dtr() and baud() methods, see my PR for an example sketch that I believe essentially does the same (and even adds break support).

[NicoHood]

Could be possible, have to check that.

Wouldnt it also make sense to mark the USB functions as weak, so one can overwrite the USB clock settings etc? Then other MCUs can be supported with a library via a strong implementation which overwrites those functions. Or even with a .cpp file in the pins_arduino.h

This would help if the Arduino team does not want to include my u2 support code to the core itself (if there is any reason for this, dunno).
USB_ClockEnable() and Disable()

[NicoHood]

Should I open a new PR with this branch?
https://github.com/arduino/Arduino/compare/master...NicoHood:USB-Core-Fixes?expand=1

I had so many problems to get my git in sync with the master, I am not able to repair this PR, so I'd create a new one if desired. This patch has nothing complicated in it and the commits are now more clean. Please have a look.

[NicoHood]

Moved to #3640