Add variant for the ROBOTIS OpenRB-150.

[tomsrocketsandrobots]

This pull request adds support for the ROBOTIS OpenRB-150 board to the Arduino SAMD core. It includes the necessary variant files and linker scripts to enable compatibility with the SAMD21G18A microcontroller used on the board.

This change is needed to add support for the OpenRB-150 to PlatformIO. I will also be submitting a PR to this issue on platformio/platform-atmelsam to add the necessary json file to add the OpenRB-150 to platformIO's board explorer.

• Variant files copied from ROBOTIS GitHub Repository.
• OpenRB-150 Documentation: ROBOTIS e-Manual

The changes were tested using the following methods:

• Compiled and uploaded multiple sketches from the Dynamixel2Arduino library examples to the OpenRB-150 using the Arduino IDE.
• Added the variant files to my local .platformio directory, manually ported the same arduino sketches into platformio projects, and tested to confirm the sketches upload properly using platformio IDE.

Please let me know if any additional changes are required!

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1 out of 2 committers have signed the CLA.

✅ tomsrocketsandrobots
❌ ktgilliam
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