Dropping support for SAMD21 boards

[bxparks]

Back in December 2020 (just before Christmas Day if I recall), the ArduinoCore-samd project converted their underlying Arduino API to the new ArduinoCore-API. This happened on a point release, from arduino:samd 1.8.9 to 1.8.10. The new Arduino API has a number of backwards compatibility breaking changes, none of them documented as far as I know. Most of my libraries, including AceTime, broke immediately on these SAMD21 core.

I was able to continue to support some third party SAMD21 boards using the SparkFun SAMD Core. However, something changed recently, either with new Arduino IDE 1.8.19 or with the SparkFun SAMD 1.8.6. I can no longer upload binaries to my board, getting errors about not finding bossac 1.7.0.

I am now dropping support for all SAMD21 boards. I don't use them in any personal projects. I don't have any motivation for fixing this.

• All of the Arduino-branded SAMD21 boards use the new Arduino API, which are incompatible with the old API, so they are in the Blacklisted tier.
• Third party SAMD21 boards are in a new Tier 3: May work, but unsupported tier.