0254 firmware published and branch name changes

[mathias-arm]

DAPLink 0254 was released on May 15th 2019, but the firmwares were not pushed to https://armmbed.github.io/DAPLink/. This has now been done. In addition all firmwares back to 0244 (when they existed) are now available as well.

As mention in the previous announcement (#819), were are going to perform some branch renaming. The experimental_compilers branch has been renamed develop. For those that were tracking this branch, GitHub offers the following instructions:

git branch -m experimental_compilers develop
git fetch origin
git branch -u origin/develop develop

Unless an issue arises, the master branch will be renamed main early next week.