Add spartan edge libraries to Arduino Library Manager

[reivilo]

Great move @lakshan, thank you!

@lakshan

However, there is one problem. Seeed_Arduino_mbedtls library should be in dev branch to work with the new eRPC firmware along with RTL8720. The master branch is made for the old AT firmware.

I would suggest using versioning to solve this issue.

• Set version to 1.0 for current master for old AT firmware and generate a release.

• Move dev branch into master and set version to 2.0 for new eRPC firmware and generate a release.

• Add a comment on the readme.md file about the requirements for each versions.