Prepare for 1.6.12 release (Sierra related fixes)

[facchinm]

PR for @ArduinoBot

[facchinm]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5374-BUILD-600-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5374-BUILD-600-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5374-BUILD-600-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5374-BUILD-600-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-5374-BUILD-600-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.

[facchinm]

Fixes #5202, #5182, #5205, #5189, #5175, #2986, #5360, #5264

@per1234 @mperdue @descampsa and anyone interested, please test 😄

[mperdue]

#5360 has been addressed. I'm able to build the marlin firmware that I was previously unable to. I'll check it further after the parts I'm currently printing are complete.

[descampsa]

The avrdude patch from #5202 looks ok, but #5182 has not been applied, so i get the warning message when i upload the bootloader to my board (with avrdude programmer and uno board).

[per1234]

• I'm still encountering avrdude 6.3.0-arduino2 incompatible with USBasp using libusb-win32 driver avrdude-build-script#1
• lock/unlock bits fail to verify with Atmel AVRISP mkII and avrdude 6.3.0-arduino2 avrdude-build-script#2 seems to be fixed(I get the avrdude: WARNING: invalid value for unused bits in fuse "lock", should be set to 1 according to datasheet... warning).
• "Burn Bootloader" with Uno fails: "verification error ... 0xfd != 0x05" #5175 seems to be fixed(I get the avrdude: WARNING: invalid value for unused bits in fuse "efuse", should be set to 1 according to datasheet... warning).
• I can't test for Unexpected dependency on libusb.dll avrdude-build-script#3 because I wasn't able to reproduce it with Arduino AVR Boards 1.6.12(I think it was a Windows 10 specific issue).
• As with avrdude 6.3.0-arduino2, fixes Arduino IDE sending avrdude commands to avrisp2 too fast #2986 for me.
• As with avrdude 6.3.0-arduino2, fixes https://github.com/arduino/Arduino/issues/388 for Arduino as ISP(but not for USBasp with stock firmware).